XBL 2.0

Abstract

XBL (the Xenogamous Binding Language) describes the ability to associate elements in a document with script, event handlers, CSS, and more complex content models, which can be stored in another document. This can be used to re-order and wrap content so that, for instance, simple HTML or XHTML markup can have complex CSS styles applied without requiring that the markup be polluted with multiple semantically neutral div elements.

It can also be used to implement new DOM interfaces, and, in conjunction with other specifications, enables arbitrary tag sets to be implemented as widgets. For example, XBL could be used to implement the form controls in XForms or HTML.

Status of This Document

Beware. This specification is no longer in active maintenance and the Web Applications Working Group does not intend to maintain it further. No browser vendor has plans to implement this specification at this time. Unless you are interested in the history of the Web's component model, this document is likely not of any use to you.

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is the 24 May 2012 Working Group Note of XBL 2.0. Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress. The W3C Web Applications Working Group is the W3C working group responsible for this document.

If you wish to make comments regarding this document, please send them to dev-tech-xbl@mozilla.org (subscribe, archives) or public-webapps@w3.org (subscribe, archives). All feedback is welcome. The editor guarantees that all feedback sent to the above lists will receive responses.

The editor's copy of this specification is available in W3C CVS. A detailed list of changes is available from the CVS server.

This specification is a (non-backwards-compatible) revision of Mozilla's XBL 1.0 language, originally developed at Netscape in 2000, and originally implemented in the Gecko rendering engine. [XBL10]

This specification was developed by the Mozilla Foundation and its contributors, in conjunction with individuals from Opera Software ASA, Google, Inc, and Apple Computer, Inc, to address problems found in the original language and to allow for implementation in a broader range of Web browsers.

This document is also based, in part, on work done in the W3C's Bindings Task Force. However, no text from that collaboration, other than that written by the aforementioned contributors, remains in this specification. Inspiration was similarly taken from other efforts, such as HTML Components. [HTC]

Although they have had related histories, this specification is separate from the W3C's "sXBL" drafts, and is not compatible with them. (The two efforts use different namespaces, for one.)

While the body of this specification was created outside the W3C, the W3C Web Applications Working Group is now guiding this specification along the W3C Recommendation track.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

1. Introduction
2. XBL Elements
3. Binding Attachment and Detachment
4. Shadow Content
5. Binding Implementations
6. Event Handlers
7. DOM Interfaces
8. Resources
9. Summaries of Elements, Attributes, and Events
Acknowledgments
References

1. Introduction

xe·nog·a·mous adj. Of or relating to the fertilization of elements from disparate trees.

This specification defines XBL (the Xenogamous Binding Language) and some supporting DOM interfaces and CSS features. XBL is a mechanism for overriding the standard presentation and interactive behavior of particular elements by attaching those elements to appropriate definitions, called bindings. Bindings can be attached to elements using either CSS, the DOM, or by declaring, in XBL, that elements matching a specific selector are implemented by a particular binding. The element that the binding is attached to, called the bound element, acquires the new behavior and presentation specified by the binding.

Bindings can contain event handlers that watch for events on the bound element, an implementation of new methods and properties that become accessible from the bound element, shadow content that is inserted underneath the bound element, and associated resources such as scoped style sheets and precached images, sounds, or videos.

XBL cannot be used to give a document new semantics. The meaning of a document is not changed by any bindings that are associated with it, only its presentation and interactive behavior.

To help readers understand how certain features can be used, this specification includes some examples.

In these examples, a long ellipsis ("...") is used to indicate elided content that would be present in a full example but has been removed for clarity.

Here we see a simple binding being used to reorder content in an HTML page, so that the element with class="nav" is positioned before the element with class="main". CSS associated with the binding is then used to position the two elements.

<binding id="nav-then-main">
 <template>
  <style scoped>
   #wrapper { display: table-row; }
   #col1, #col2 { display: table-cell; }
  </style>
  <div id="wrapper">
   <div id="col2"><content includes=".nav"></div>
   <div id="col1"><content includes=".main"></div>
  </div>
 </template>
</binding>

The HTML page associated with such a binding might look like:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Demo</title>
  <link rel="stylesheet" href="example.css">
 </head>
 <body>
  <div class="main">
   <h1>Demo</h1>
   ...
  </div>
  <div class="nav">
   <p><a href="http://example.com/">Home</a></p>
   ...
  </div>
 </body>
</html>

The CSS stylesheet referred to from that document would include various stylistic rules, and would in particular contain a link to the XBL file, making it apply to the body element so as to reorder the two child elements:

/* Colors and Fonts */
h1 { font: 2em sans-serif; color: green; background: white; }
...

/* Reorder content */
body { binding: url(example.xbl#nav-then-main); }

The result of all the above is equivalent to the result one would get if one simply placed the div element with class="nav" before the div element with class="main". However, the effect is achieved without needing any changes to the markup. This allows the same markup to be given different presentations dynamically. It also allows changes to be applied across entire sites by merely changing global stylesheet and binding files, much as CSS can be used to change the layout and presentation of a site even without XBL.

Here is another example, this time of an inaccessible implementation of the proposed HTML5 details disclosure element: it opens and closes when clicked, and reflects its current state in the element's "open" attribute.

<binding element="details">
 <template>
  <style scoped>
   #container[data-state=hidden] { display: none; }
  </style>
  <div>
   <div><content includes="summary:first-child">Details...</content></div>
   <div data-state="hidden" id="container"><content></content></div>
  </div>
 </template>
 <implementation>
  ({
    get open() { return this.boundElement.hasAttribute('open'); },
    set open(val) {
      if (val)
        this.boundElement.setAttribute('open', 'open');
      else
        this.boundElement.removeAttribute('open');
      return this.open;
    },
    xblBindingAttached: function () {
      this.addEventListener('click', function () { this.open = !this.open }, false);
      this.addEventListener('DOMAttrModified', function (event) {
        if (event.attrName = 'open') {
          if (event.attrChange = MutationEvent.ADDITION) {
            this.shadowTree.getElementById('container').dataset.state = 'visible';
          } else if (event.attrChange = MutationEvent.REMOVAL) {
            this.shadowTree.getElementById('container').dataset.state = 'hidden';
          }
        }
      }, false);
    },
  })
 </implementation>
</binding>

Since the examples are all untested (there are no XBL2 implementations at the time of writing), it is quite possible that they have errors. Please report any errors you think you see, so that we can correct the examples.

1.1. Editor's note

This specification was updated (as of Septemebr 2010) to make the following broad changes:

Moving XBL to just be part of HTML, rather than having its own namespace and requiring XML.
Removing a number of features so as to reduce the initial implementation burden: handlers (events forward to the internal object now, and that object implements EventTarget); style and script are replaced by HTML equivalents; prefetch and handler are removed altogether; removed support for namespaces.
Making loadBindingDocument() asynchronous.
Adding an API to allow XBL to be manipulated from script.

This specification is not written in the strictest style; in many places it is rather hand-wavy and fails to define things strictly (e.g. in terms of event loops, WebIDL, and other such mechanisms that allow us to define the Web platform strictly). If this update to XBL2 is more successful than the previous draft, then it will likely be merged into the HTML specification, and the processing models will be rewritten to be much more well-defined. In particular, this will include making binding get parsed in head elements, and the syntax being updated to define that you can't use content or inherited in places that are handled in an insertion mode other than "in body".

1.2. Relationship to XBL1

This specification is not backwards compatible with XBL1.

There are numerous changes. However, of particular importance to readers familiar with XBL1, there have been some changes to the element names. In particular, the XBL1 element content is now called template, and the XBL1 element children is now called content.

1.3. Relationship to XSLT

This specification has a similar scope to XSLT. The main differences are:

XSLT operates on a static DOM, permanently replacing that DOM for rendering. XBL, on the other hand, transparently transforms the DOM for rendering while leaving the underlying structure intact, and dynamically reflects changes to the underlying DOM in the transformed rendering.
XSLT allows any arbitrary transformation to be performed. XBL shadow trees, on the other hand, only support reordering of the bound element's child nodes and interleaving of those explicit children with shadow content. Arbitrary transformations are not possible in XBL while retaining the transparent nature of XBL's shadow tree processing.

In addition, XBL can be used for component creation, which is not covered by XSLT.

1.4. Terminology and Conventions

An XBL user agent is an implementation that attempts to support this specification.

A binding is the definition of behavior that can be applied to an element so as to augment its presentation.

The namespace of all the XBL elements must be the same as HTML: http://www.w3.org/1999/xhtml

The term binding document is used to mean a Document containing one or more binding elements.

Here is a binding document. The binding is highlighted.

<!DOCTYPE HTML>
<html>
 <head>
  <title>Demo</title>
  <script>
    function fmt(n) {
      if (n < 10)
        return "0" + n;
      else
        return n;
    }
  </script>
  <binding element=".date">
   <implementation>
     ({
       xblBindingAttached: function() {
         var tm = /(\d\d\d\d)-(\d\d)-(\d\d) (\d\d):(\d\d) UTC$/.exec(this.boundElement.textContent);
         var date = new Date();
         date.setUTCFullYear(parseInt(tm[1], 10));
         date.setUTCMonth(parseInt(tm[2], 10) - 1);
         date.setUTCDate(parseInt(tm[3], 10));
         date.setUTCHours(parseInt(tm[4], 10));
         date.setUTCMinutes(parseInt(tm[5], 10));
         date.setUTCSeconds(0);
         this.boundElement.textContent = date.getFullYear() + "-" +
                                     fmt(date.getMonth() + 1) + "-" +
                                     fmt(date.getDate()) + " " +
                                     fmt(date.getHours()) + ":" +
                                     fmt(date.getMinutes()) + " LT";
         this.boundElement.title = "Adjusted to local time zone"
       },
     })
   </implementation>
  </binding>
 </head>
 <body>
  <h1>Demo</h1>
  <p class="date">2006-08-10 18:40 UTC</p>
  <p>...</p>
 </body>
</html>

(As an aside, the binding defined in this example causes elements with class="date" to have their content parsed into a UTC date and converted into a local time. The binding mutates the original DOM to do this, and it doesn't reflect any dynamic changes made to the element's content; there are better, albeit slightly more involved, ways of achieving the same effect that don't have these problems.)

A bound element is an XML or HTML element to which a binding has been applied.

In the example above, the first p element is the bound element.

A bound document is an XML or HTML document containing one or more bound elements.

In the example at the top of this section, the document is both the binding document (because it contains the definition of the binding), and the bound document (because it contains the affected bound element). In the example in the introduction section, the HTML file is the bound document, and the XBL file is the binding document.

In this specification, the term in error, typically used of an element or attribute, means that the element, attribute, or other construct is not conformant according to the rules of this specification. Rules for exactly how the construct must be treated when it is in error are always given when the term is used. Typically this will involve ignoring the erroneous nodes, meaning the UA must, for the purposes of XBL processing, act as if those nodes were absent. UAs must not, however, remove such nodes from the DOM in order to ignore them, nor should it change what DOM interfaces those nodes implement. The nodes retain all their non-XBL semantics.

UAs should report all errors to users, although they may do this in an unobtrusive way, for example in an error console.

In addition to the error handling rules given in this specification, UAs may abort all processing when encountering an error.

Aborting is only likely to be a viable error handling mechanism in controlled environments, e.g. in conformance checkers. Web browsers are expected to use the error recovery mechanisms described in this specification, not abort.

A correct element, attribute, value, or binding is one which is not in error.

The following fragment has a content element that is in error because its includes attribute has an invalid selector:

<binding id="demo">
 <template>
  <content includes="@"></content>
 </template>
</binding>

The term "semantics" is used to refer to the intrinsic meaning or processing model of elements, attributes, events, and DOM interface members. Semantics are defined by specifications; for example, this specification defines the semantics of XBL elements.

When this specification refers to elements in a namespace, it does not exclude elements in no namespace; the null namespace is considered a namespace like any other for the purposes of XBL processing.

All element names, attribute names, and attribute values in XBL are case sensitive, with the exception of attribute values defined by other specifications (those have the sensitivity defined by those other specifications).

The HTML parser lowercases all elements and attribute names, so it can appear like XBL is case-insensitive when used in HTML. However, that's actually a feature of the HTML parser, not of XBL.

An XML MIME type is text/xml, application/xml, or any MIME type ending with the string +xml (ignoring any MIME parameters).

The terms "author style sheets", "user style sheets", "user agent style sheets", and "pseudo-element" are used as defined by the CSS specifications. [CSS21]

1.5. Conformance

As well as sections marked as non-normative, all diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]

There are two classes of products that can claim conformance to this implementation: binding documents, and XBL user agents.

Binding documents must satisfy the constraints described in this specification in order to be considered conformant.

Products that generate binding documents cannot claim conformance to this specification, though they can claim to only produce binding documents that themselves are conformant to this specification.

XBL user agents must behave as described by this specification in order to be considered conformant, even when faced with non-conformant content.

User agents may optimize any algorithm given in this specification, so long as the end result is indistinguishable from the result that would be obtained by the specification's algorithms. (The algorithms in this specification are generally written with more concern for clarity than over efficiency.)

This specification is defined in terms of the DOM. The language in this specification assumes that the user agent expands all entity references, and therefore does not include entity reference nodes in the DOM. If user agents do include entity reference nodes in the DOM, then user agents must handle them as if they were replaced by their DOM replacement values when implementing this specification.

For example, if a requirement talks about an element's child text nodes, then any text nodes that are children of an entity reference that is a child of that element would be used as well.

1.5.1. Error Handling

This specification describes the rules for processing of XBL elements and related features, whether they are used in a conformant manner or not. Conformant implementations, therefore, will interoperably handle any content, whether valid or not.

1.5.2. Attributes Containing Selectors

The element attribute of the binding element and the includes attribute of the content element, if specified, must have their values parsed according to the rules in the Selectors specification. [SELECTORS]

This specification does not specify what level of Selectors support is required.

Namespace prefixes cannot be used with selectors in XBL, as there is no way to declare the prefixes. The default namespace in selectors in XBL attributes is always unbound.

The following fragment defines a binding that is bound to elements declaring links (e.g. the a element in HTML).

<binding element=":link, :visited">
 ...
</binding>

This example defines a binding that matches elements with the name a, regardless of what namespace they are in. This would match both the HTML a element and the SVG a element.

<binding element="a">
 ...
</binding>

1.5.3. Attributes Containing Space-Separated Values

Some attributes are defined as taking space-separated values. The list of values for such attributes must be obtained by taking the attribute's value, replacing any sequences of U+0009, U+000A, U+000D, and U+0020 characters (in any order) with a single U+0020 SPACE character, dropping any leading or trailing U+0020 SPACE character, and then chopping the resulting string at each occurrence of a U+0020 character, dropping that character in the process.

A space-separated attribute whose value is the empty string, or which consists of only U+0009, U+000A, U+000D, and U+0020 characters, has no values.

In the attribute attributes="1 2", the values are "1" and "2".

In the attribute class=" key - note - rocks", there are five keywords: "key", "note", "rocks", and two occurrences of the single-character keyword "-".

1.5.4. Attributes Containing URIs

Some attributes, pseudo-attributes, and method arguments are defined as specifying URIs. Such attributes must have values that match the URI token of RFC 3986 or the IRI token of RFC 3987. If they do not, they are in error (though the processing of erroneous URIs varies depending on the context). [RFC3986] [RFC3987]

1.5.5. Attributes Containing Keywords

Certain attributes are defined as requiring certain values, e.g. true or false. For such attributes, the values must be specified exactly, in the case given in this specification, and with no leading or trailing whitespace. Implementations must only perform literal comparisons, and must not use case-insensitive comparisons nor trim attribute values before comparison.

1.6. Security Concerns

This section is non-normative.

XBL raises a number of security concerns.

Data theft: A naïve implementation of XBL would allow any document to bind to bindings defined in any other document, and (since referencing a binding allows full access to that binding document's DOM) thereby allow access to any remote file, including those on intranet sites or on authenticated extranet sites. XBL prevents this by diallowing cross-origin bindings.

Privilege escalation: In conjunction with data theft, there is the concern that a page could bind to a binding document on a remote site, and then use the privileges of that site to obtain further information. XBL prevents this by requiring that the bindings all run in the security context of the bound document, so that accessing a remote binding document does not provide the bound document with any extra privileges on the remote domain.

Cookie theft: Related to privilege escalation is the risk that once an access-controlled binding document hosted on a remote site has been loaded, authentication information stored in cookies for that domain would become accessible to the bound document. XBL prevents this by requiring that the cookie attribute on the HTMLDocument interface be set to null.

Secure bindings: Using XBL for bindings that need access to the local filesystem, e.g. for implementing File Upload form controls, is not yet handled by this specification. However, a future version will provide a secure way to define an XBL binding that can be used to implement privileged mechanisms that can then be used by other bindings to provide such controls.

2. XBL Elements

When an XBL element is found inside an element other than those listed under the "Expected contexts" list in the definitions below, it is in error. When an XBL element has a child node that does not satisfy the "Expected children" list in its definition (for instance because it is the wrong node type, wrong element type, or because too many elements of its type preceded it), the child is in error. In both cases, being in error means that the UA must, for the purposes of XBL evaluation, treat the element and its descendants as it would if the erroneous node and all its descendants were not present in the DOM.

However, non-XBL elements retain their semantics, even when considered to be in error for the purposes of XBL.

Regardless of the requirements of the last few paragraphs and of the "expected children" lines, comment nodes, and text and CDATA nodes containing only whitespace characters, may always be given as children of XBL elements.

For cases where attributes on XBL elements do not conform to this specification, the error handling is similar: the attributes must be considered to be in error and the UA must ignore them, meaning that the presence of these non-conforming attributes in no way affects the XBL processing.

Further error handling rules for more specific cases are given where appropriate.

XBL user agents that support CSS should act as if they had the following rules in their UA style sheet:

@namespace html url(http://www.w3.org/1999/xhtml);
html|binding, html|implementation, html|template, html|content, html|inherited { display: none; }

XBL user agents that do not support CSS should not render XBL elements.

The following sections describe the content model of XBL elements, but not their actual processing model. The processing model for XBL is described in later sections.

2.1. The `binding` Element

Expected context:: Metadata content (e.g. an HTML head element)
Expected children (in any order):: implementation: zero or one.; template: zero or one.

The binding element describes a single XBL binding that adds presentation and interactive behavior to XML or HTML elements. Each binding has these optional components:

Methods, Properties, and Fields: A binding can specify additional methods that can be invoked on the element. It can also specify additional properties and fields that can be retrieved or set on the element. In this way the functionality of the bound element becomes extensible. (See: binding implementations.)

Template: The optional template defines the initial shadow content for the bound element.

Bindings can act as an attachment mechanism, specifying elements to associate with the given binding when the binding is imported, using the element attribute.

Attributes

Global attributes

Any of HTML's global attributes.

extends

The extends attribute is used to specify the URI of a binding that this binding inherits from. (See: interpretation of URIs to XBL bindings.) If the URI is in error or does not refer to another binding, the UA must ignore it, meaning that this binding does not explicitly inherit from another binding. (See: explicit inheritance.) Only one URI can be specified.

element

This attribute, if specified, must contain a selector. All elements in the binding document, and in any documents that import the binding document, that match the given selector, must be bound to the binding defined by this binding element. (The element's own shadow tree, if any, must not be taken into account when determining if it matches a selector for the purposes of this attribute.)

If an element attribute contains an invalid selector, it is in error and must be ignored, meaning that while the binding is still parsed and may be referenced using other attachment mechanisms, the binding is not attached to any element by its element attribute, as if the attribute had simply been omitted.

The binding element defines a presentation and behavior binding. It does not define an element's semantics. If an element has no semantics when processed alone, then it has no semantics when processed with XBL.

Sending markup that does not have well-defined semantics over the network is bad practice. XBL is intended to be used to augment the user experience, for instance by providing better quality widgets or enhancing aesthetics. If the document being sent is unusable without XBL, then XBL is being abused.

This binding extends a binding defined in an external file. The binding in the other file defines a value property, and fires events when that property is changed. This binding just implements a check box that all div elements with the class "checkbox" will be bound to.

<binding element="div.checkbox" id="checkbox"
         extends="http://www.example.org/resources/ui-core.xml#valuedControl">
 <template>
  <style scoped>
   #wrapper > div { display: inline-block; }
  </style>
  <div id="wrapper">
   <div id="control"></div>
   <div id="label"><content></content></div>
  </div>
 </template>
 <implementation>
  ({ xblBindingAttached: function () {
       this.addEventListener('click', function () {
         if (this.baseBinding.value == 'on')
           this.baseBinding.value = 'off';
         else
           this.baseBinding.value = 'on';
       }, false);
       this.addEventListener('change', function () {
         if (this.baseBinding.value == 'on')
           this.shadowTree.getElementById('control').textContent = '☑';
         else
           this.shadowTree.getElementById('control').textContent = '☐';
       }, false);
     } })
 </implementation>
</binding>

2.2. The `implementation` Element

Expected context:: binding
Expected children:: If the element has no src attribute: JavaScript text.; If the element does have a src attribute: none.

The implementation element describes a set of methods, properties, and fields that are attached to the bound element. Once the binding is attached, these methods, properties, and fields can be invoked directly from the bound element.

The implementation element, if present, must either contain JavaScript code, or have a src attribute that points to a JavaScript file. (See: binding implementations.)

Attributes

Global attributes: Any of HTML's global attributes.
src: The src attribute specifies the URI to a JavaScript resource. If the attribute is specified, the contents of the element must be ignored (even if the resource could not be fetched or was of the wrong type). (See: binding implementations.)

If the implementation element points (using the src attribute) to a resource that is unavailable, then it is in error and the UA must ignore it, meaning it must not be used as an implementation definition for any binding.

implementation blocks are evaluated once, on first use. Changes to an implementation element or its contents have no effect once the element has been evaluated. (See: loading and running scripts, binding implementations.)

The following example shows a binding that defines a new method, two new properties (one with a custom getter and setter, one without), an internal field (used to back the property), and some hooks to initialize the binding and to handle the bound element being inserted and removed from the document. The binding applies to any element with the class "demo" (in the files into which it is imported, that is).

<binding element=".demo">
  <implementation>
   ({
      add: function (op1, op2) {
        return op1+op2;
      },
      get memory() {
        return this._memory.toString();
      },
      set memory(value) {
        this._memory = parseInt(value, 10);
      },
      xblBindingAttached: function() {
        this._memory = 0; // internal property to back "memory" external property
        this.external.state = 'initialized'; // external property
      },
      xblEnteredDocument: function() {
        this.external.state = 'in document';
      },
      xblLeftDocument: function() {
        this.external.state = 'out of document';
      },
   })
  </implementation>
</binding>

The following example shows how to refer to an external file for the implementation of a binding. This is the same binding as the previous example.

<binding element=".demo">
 <implementation src="demo.js"></implementation>
</binding>

The demo.js file would look like this:

({
   add: function (op1, op2) {
     return op1+op2;
   },
   get memory() {
     return this._memory.toString();
   },
   set memory(value) {
     this._memory = parseInt(value, 10);
   },
   xblBindingAttached: function() {
     this._memory = 0; // internal property to back "memory" external property
     this.external.state = 'initialized'; // external property
   },
   xblEnteredDocument: function() {
     this.external.state = 'in document';
   },
   xblLeftDocument: function() {
     this.external.state = 'out of document';
   },
})

2.3. The `template` Element

Expected context:: binding
Expected children:: Anything. Of particular interest, the content and inherited elements may occur as descendants, and non-XBL descendant elements may host attributes and pseudo attributes.

The template element contains arbitrary element descendants. When a binding is attached, the template element's child nodes are cloned and attached to the bound document under the bound element. Dynamic changes to the descendants of template elements are reflected in bindings. (See: shadow content.)

Attributes

Global attributes: Any of HTML's global attributes.
apply-author-sheets: The apply-author-sheets attribute indicates whether or not rules in author style sheets associated with the bound element's document apply to the shadow content generated by the binding. Its value must be either true (indicating that they do) or false (indicating that they do not). The default behavior, which is used when the attribute is omitted or has a value other than the two allowed values, is to not apply the bound document's author style sheets (same as false). (See: binding style sheets.)
allow-selectors-through: The allow-selectors-through attribute indicates whether or not rules in CSS can cross scopes. Its value must be either true (indicating that they can) or false (indicating that they cannot). The default behavior, which is used when the attribute is omitted or has a value other than the two allowed values, is to not let selectors cross scopes (same as false). (See: selectors and shadow scopes.)

The semantics of non-XBL elements inside this element are untouched, which can lead to unintuitive results. (See: semantics of non-XBL elements in XBL contexts.)

The following binding defines a shadow tree that wraps the contents of the bound element in four blocks. It uses the apply-author-sheets attribute to allow the bound document to style the nodes directly, and uses the allow-selectors-through attribute to allow the bound document to pretend (for the purposes of selector matching) that the shadow tree actually is descended from the bound element.

<binding id="wrapBy4">
 <template apply-author-sheets="true" allow-selectors-through="true">
  <div class="wrap1">
   <div class="wrap2">
    <div class="wrap3">
     <div class="wrap4">
      <content></content>
     </div>
    </div>
   </div>
  </div>
 </template>
</binding>

Using this binding could take the following document:

<DOCTYPE HTML>
<html>
 <head>
  <title>Pretty Title</title>
  <style>
   h1 span { display: block; }
   h1 { border: solid red; }
   h1 .wrap1 { border: solid orange; }
   h1 .wrap2 { border: solid yellow; }
   h1 .wrap3 { border: solid green; }
   h1 .wrap4 { border: solid blue; }
  </style>
 </head>
 <body>
  <h1>
   <span class="wrap1">
    <span class="wrap2">
     <span class="wrap3">
      <span class="wrap4">
       Pretty Title
      </span>
     </span>
    </span>
   </span>
  </h1>
  ...
 </body>
</html>

...and shrink it to this:

<DOCTYPE HTML>
<html>
 <head>
  <title>Pretty Title</title>
  <style>
   h1 { binding: url(cool.xml#wrapBy4); }
   h1 { border: solid red; }
   h1 .wrap1 { border: solid orange; }
   h1 .wrap2 { border: solid yellow; }
   h1 .wrap3 { border: solid green; }
   h1 .wrap4 { border: solid blue; }
  </style>
 </head>
 <body>
  <h1>Pretty Title</h1>
  ...
 </body>
</html>

...which removes the semantic-free elements used as presentation hooks from the content markup layer, and places them in the presentation layer where they belong.

2.4. The `content` Element

Expected context:: Any, but there must be a correct template element somewhere in the ancestor chain, and there must not be any content elements anywhere in the ancestor chain.
Expected children:: Anything.

The content element is used inside shadow content to specify where explicit children that might already exist underneath the bound element are inserted into the shadow tree. As far as the presentation model is concerned, any shadow content the binding places between the bound element and the content elements is interleaved between the bound element and its explicit children without affecting the document model.

If the includes attribute has children of the bound element assigned to it, then those children are inserted into the final flattened tree in place of the content element. Otherwise, the child elements of the content element are inserted into the final flattened tree in place of the content element instead. (See: processing content elements.)

Attributes

Global attributes: Any of HTML's global attributes.
includes: The includes attribute can be used to indicate that only certain content should be placed at the content element. Its value must be a valid selector. (See: processing content elements.)
apply-binding-sheets: The apply-binding-sheets attribute indicates whether or not scoped style sheets loaded for an XBL binding are applied to a bound element's explicit children (in addition to the bound element itself) that are inserted below this content element when it is processed. Its value must be either true (indicating that they are) or false (indicating that they are not). The default behavior, which is used when the attribute is omitted or has a value other than the two allowed values, is that they are not applied (same as false). (See: binding style sheets.)
locked: The locked attribute indicates whether or not new children may be inserted below this content element when it is processed. Its value must be either true (indicating that they may not) or false (indicating that they may). The default behavior, which is used when the attribute is omitted or has a value other than the two allowed values, is that they may be inserted (same as false). Elements already assigned to a content element whose locked attribute is dynamically changed are not removed from that element. (See: processing content elements.)

This sample extract from a binding document shows how to use the includes attribute to distribute children to different parts of a shadow content template.

<binding element=".grid">
 <template>
  <div class="caption" attributes="text=title"></div>
  <div class="outer-table">
   <div class="columns">
    <content includes=".column">
     <!-- default to have just one column if none are declared -->
     <div class="column"></div>
    </content>
   </div>
   <div class="rows">
    <content includes=".heading"></content>
    <div class="body">
     <content includes=".row:not([hidden])"></content>
    </div>
   </div>
  </div>
 </template>
 ...
</binding>

The above template would be used with markup such as the following:

<div class=grid title="The Lesser of Two Evils">
 <div class=column id="product" sort="alphabetic primary">

<div class=heading> <div class=item>Product<div> <div class=item>Catchphrase<div> <div> <div class=row> <div class=item>Arachno Spores<div> <div class=item>The fatal spore with the funny name<div> <div> <div class=row> <div class=item>Pastorama<div> <div class=item>Located on the former site of Brooklyn<div> <div> <div>

2.5. The inherited Element

The inherited element represents where the next inherited shadow tree is to be inserted. If the binding is the base binding (and thus has no inherited bindings) or if none of the bindings it inherits from have shadow trees, or if this is not the first inherited element in the binding's shadow tree, then the contents of the inherited element (if any) will be used instead. (See: the final flattened tree.)

Attributes

While it is legal to nest inherited elements, it is pointless, since if one inherited element used its fallback content, any subsequent such elements will too.

2.6. The attributes Attribute

The attributes attribute is a global attribute that specifies which attributes on the bound element should be forwarded to the element on which the attribute is found when the shadow content template is cloned. It is a space-separated list of pairs of attribute names separated by equal signs, each possibly suffixed by a hash character ("#") and a type designation. (See: attribute forwarding.)

2.7. The pseudo Attribute

The pseudo attribute is a global attribute that specifies the pseudo-element that, when used on the bound element, must be mapped to the element on which the attribute is found.

The value of the pseudo attribute must be a valid pseudo-element name, in lowercase, without the leading "::". The valid pseudo-elements are defined by the CSS specifications, and are "value", "choices", "label", "repeat-item", and "icon". Future versions of CSS might introduce new values. (See: matching pseudo-elements.)

3. Binding Attachment and Detachment

Bindings must be attached as soon as the following conditions have all been met:

In particular, binding can happen before the element is inserted into its document.

If the binding document was already loaded when the element was created, or when it became known that the element matched the binding (e.g. because the binding's element attribute is mutated in a script), then the binding must be applied such that to any running scripts, notwithstanding those implementing xblBindingAttached() notification methods, it appears that the binding was applied immediately. When mutation events are to fire, they must fire after the binding being applied.

If the binding document has yet to be (fully) loaded when it becomes known that the binding applies, then the user agent must wait until all running scripts have completed before attaching the binding.

If the binding attachment mechanism is the 'binding' property, then it does not become known to the user agent that the binding applies (or does not apply) until the next time style resolution is performed. This specification does not define when style resolution happens.

Bindings must be detached as soon as it is known that the binding no longer applies to the element. In the case of attachment using the 'binding' property, this won't be until the next time style resolution is performed.

When it becomes known that a binding is to be detached, it must happen such that to any running scripts it appears that the binding was removed immediately, except if the script in question is running as part of the last step of the binding attachment process, in which case the detachment happens after all the bindings being attached have had their methods called. (See: binding attachment model.)

3.1. The Bindings-Are-Ready State

XBL implementations must have a bindings-are-ready counter (for each Document) that keeps track of when there are outstanding bindings to bind. This counter is set at the times given in the following sections. (It is not set for all kinds of attachments.)

If the bound document is being parsed by the user agent, then, as soon as all the following conditions have been met, the user agent must fire an xbl-bindings-are-ready event on the document's root element. The conditions are:

The event must use the Event interface, must bubble, must not be cancelable, and must not have a default action. [DOM3EVENTS]

The bound document's load event must not fire until after the xbl-bindings-are-ready event has fired. The xbl-bindings-are-ready event must fire even in the complete absence of any use of XBL in the document.

3.2. Attachment using <binding element="">

While an element matches the element attribute of one or more of the binding elements that is imported into, or defined in, the element's document, the bindings defined by each such binding element must be bound to the element. This applies to all elements that are associated with a document, even when they have not yet been inserted into the document, or are not in the final flattened tree.

3.2.1. Importing Binding Documents

The <?xbl?> processing instruction specifies a binding document to load. Any bindings defined in that document must be applied to matching elements in the document with the processing instruction.

<?xbl?> processing instructions that occur after the root element's start tag in the markup are in error. <?xbl?> PIs that are dynamically inserted through the DOM after the root element's start tag has been parsed or the root element has been attached to the document are in error too.

A <?xbl?> processing instruction that is not in error according to the above must be parsed using the same syntax as the XML Stylesheet PI. [XMLSSPI] If there are any parse errors, then the entire processing instruction is in error.

Otherwise, if it has an href pseudo-attribute then it specifies the URI of the binding document to import. If the URI cannot be resolved, or returns an error, or does not point to a resource with an XML MIME type, or has any other problem that makes it unusable, then the processing instruction is in error.

If a processing instruction is in error (as described in the previous few paragraphs) then it must be ignored.

Otherwise, the referenced document must be loaded (unless it has already been loaded), and any bindings defined by that document must be applied to matching elements in the document that contained the PI. Once loaded, the binding document is added to the bindingDocuments list of the document with the PI.

Dynamic changes to <?xbl?> processing instructions must be ignored from an XBL standpoint.

For example, if new binding elements are added to an imported binding document (via the DOM), then the new bindings are immediately applied to the document that had the PI importing that binding document.

XBL bindings are always implicitly imported into the document in which they are defined.

Whenever an <?xbl?> PI causes a binding document to be loaded, the document's bindings-are-ready counter must be increased by one. Once the binding document is loaded, the counter must be decreased by one.

3.3. Attachment using CSS

Bindings can be attached to elements through CSS using the 'binding' property.

Bindings attached through CSS must only remain on the bound element as long as the element continues to match the style rule. If at any time a resolution of style on the element determines that a different binding should be attached, the old binding (and all bindings that it explicitly extends in its explicit inheritance chain) must be detached.

Whenever an element is removed from a document, any bindings attached to that element via CSS must be detached.

Attaching a binding using CSS does not import the binding document. The element attributes of binding elements in the binding document do not take effect unless the binding document is imported. (See: importing binding documents.)

3.3.1. The 'binding' Property

'`binding`'
Value:	none \| [ <uri> ]* <uri>
Initial:	none
Applies To:	all elements (but not pseudo-elements)
Inherited:	no
Percentages:	n/a
Media:	all
Computed Value:	specified value, with URIs resolved to absolute URIs

3.3.2. Processing Model

User agents may perform the CSS cascade, inheritance, and computation stages either across the entire tree, or per element, or per property per element, and either before applying bindings, or simultaneously, while applying bindings. In any case, for each element the computed value of 'binding' must be found and then used to apply the bindings to the element (when the element is first styled, and each subsequent time the styles that match the element change).

Since each time a binding is applied it can change the computed values of properties of elements that are descendants of the bound element, this may require several passes. This may be avoided by computing the value of the 'binding' property for the element, and then applying any bindings, before any of its descendants.

3.3.3. Examples

3.4. Attachment using the DOM

Bindings can be attached to elements through the DOM using the ElementXBL interface. The method addBinding takes a binding URI and attaches the binding to the element.

This attachment is not necessarily synchronous. Scripts that invoke this method should not assume that the binding is installed immediately after this method returns. (An xbl-bound event is fired when the binding is installed.)

When a binding is attached using the DOM, it inherits from the current most derived binding that is already attached to the element, if any. (See: binding inheritance.)

Any bindings attached to an element using the addBinding() method will remain on the element until the element is destroyed or a corresponding removeBinding() call is made.

3.5. Binding Attachment Model

When a new binding is attached, the UA must perform the following steps in order (or act as if it did). Implementations may choose to suspend redraw during this process.

The attachment process for the binding must then wait for the above steps to have been completed for all bindings that are known to apply to elements. When all the new bindings have reached this point, then, for each newly attached binding, the xblBindingAttached() method must be invoked on the binding's implementation, immediately followed, if that bound element is in a document, by the invocation of the xblEnteredDocument() method.

The order that bindings on different bound elements have these methods called must be the relative tree order of all their bound elements, as returned by the compareDocumentPosition() function. In certain cases (e.g. bound elements in disconnected fragments), this order is implementation-specific; however, it must always be consistent with the return values of that function. [DOM3CORE]

The order that bindings on the same bound element have these methods called must be the derivation order, with less derived bindings being initialized before more derived bindings.

After all the appropriate methods have been called, an xbl-bound event that bubbles, is not cancelable, has no default action, and uses the Event interface, must be fired on every bound element that just got bound, in the same order as their xblBindingAttached() methods were invoked. (See: binding inheritance.) [DOM3EVENTS]

If a binding stops applying to a document while the above steps are being applied, the binding is not removed until after the steps above have all been completed. Once they have been completed, any bindings that no longer apply must be detached. (See: binding detachment model.)

3.6. Handling Insertion and Removal from the Document

A bound element is in a document if it has a Document node as an ancestor, or it is in a shadow tree and that shadow tree's bound element is itself in a document.

These methods must be invoked as soon as the DOM is in a stable state, after any mutation events have fired, and after all running scripts have finished executing. If a bound element is removed and then reinserted into a document (or vice versa) during script execution, or while mutation events are being fired, the user agent must coalesce all the notifications into zero or one method calls (i.e. matching pairs of insertions and removals must not cause bindings to be notified).

3.7. Binding Inheritance

Bindings can inherit from each other explicitly using the extends attribute. They can also inherit from each other implicitly if multiple bindings are attached to an element.

An explicit inheritance chain is a chain of bindings connected using the extends attribute.

An implicit inheritance chain is a chain of explicit inheritance chains. There can be at most one implicit inheritance chain per bound element.

A base binding is a binding that does not inherit from any other binding, either explicitly or implicitly. The base binding of a bound element is at one end of the bound element's implicit inheritance chain.

A base binding of the explicit chain is any binding that does not inherit explicitly from another, but may inherit implicitly from other bindings.

A most derived binding is a binding that no other binding inherits from. The most derived binding of a bound element is the binding at the other end of the bound element's implicit inheritance chain from the base binding.

If B_i is the ith binding in a group of N chains, where B₁ is the base binding and B_N is the most derived binding, then B_i-1 is the next most derived binding of B_i.

In this specification, inheritance is represented as an arrow pointing to the binding that is being inherited. Thus, in the chain A→B, the A binding is the most derived binding, and the B binding is the next most derived binding, in this case also the base binding.

3.7.1. Explicit Inheritance

If a binding element's extends attribute is changed, then, for each time the binding is bound to an element, the user agent must follow these steps:

It is possible to form a loop with the extends attribute. For example, a binding A can inherit from B which inherits from C which inherits from B again. The attachment algorithm is defined in a way that makes the loop stop as soon as a duplicate binding would be bound. In this case, the user agent will form a chain starting with A (the most derived binding), derived from B, derived from C, with C as the base binding (chain A→B→C). If, given the same definitions, the element was bound directly to C, then the chain would be C→B.

3.7.2. Implicit Inheritance

When two bindings are both attached to the same element, the base binding at the end of the explicit inheritance chain of the second binding implicitly inherits from the most derived binding of the explicit inheritance chain of the first.

The order of bindings is always such that bindings added via the binding element are first (in the order the bindings are specified in the file, with the files, if there are more than one, ordered in the same order that they are referred to, traversed pre-order, depth-first), the bindings attached via CSS are second (in the order specified on the 'binding' property), and the bindings added via addBinding are third (in the order they were attached, most recently attached being the most derived binding).

An element can be bound to the same binding multiple times, in which case a binding can end up inheriting from itself. (This can only happen via implicit inheritance, though.)

3.7.3. Mixing Implicit and Explicit Inheritance

Output:	H	e	l	l	o	-	W	o	r	l	d	!
Source:	c	b	a	a	a	a	a	b	b	c	R	c

3.8. Attachment During Document Load

Binding loads are asynchronous. That is to say, when a binding is added (either via style sheet, script, or some other method), and the relevant binding document is not yet loaded, the load must be started in the background and the binding only attached once the binding document is available. An author can ensure that all bindings are synchronously attached by calling loadBindingDocument to pre-fetch any binding documents that are required.

The bound document must wait until all XBL dependencies have loaded before firing its load event.

3.9. Binding Detachment Model

When a binding is detached, the xblLeftDocument() method must be invoked on the binding's implementation. Then, the shadow tree must be removed, the implementation must be removed from the bound element's list of binding implementations, and any forwarding of events to the binding must be stopped for this bound element.

If the binding had an extends attribute when it was bound to the element (it may have changed since then, in which case the binding is being detached precisely for that reason), then the user agent must then detach the binding that was attached because of that attribute (if any). (See: explicit inheritance, interpretation of URIs to XBL bindings.)

If the binding had a shadow tree, the explicit children must then be redistributed. (See: processing content elements.)

4. Shadow Content

The subtree specified by the template element is referred to as the shadow content template. This template describes a content tree that will be generated under the bound element during binding attachment. An element declared in a bound document using a single element can then be constructed out of multiple child elements, and this implementation is hidden from the bound document.

A shadow tree is a tree of nodes created by cloning a binding's shadow content template. A bound element can have zero, one, or more shadow trees. If a bound element has any, they are combined by the user agent, along with the element's explicit children, to form the final flattened tree. Shadow trees are hidden from normal DOM processing (hence the name "shadow"); they are not accessible via Core DOM navigation facilities such as firstChild or nextSibling. (See: shadow content.)

The term shadow content refers to the various nodes in the shadow tree(s) of a bound element. Shadow content is created by cloning shadow content templates during binding attachment.

The file control is actually a special case. Due to security considerations, untrusted bindings will typically not be able to extend the file upload control in UAs intended for use with untrusted content.

Whenever bindings are attached to an element, shadow content will potentially be generated. Similarly, when a binding is removed from an element, its shadow content, if any, is destroyed.

A shadow tree must be generated if a template element is added to a binding element that had no template element. If the template element is removed, then the shadow tree must be destroyed.

The template element used to generate a binding is always the first such element in a binding element. If the binding element is mutated in a way which changes which template element is the first, then the corresponding shadow tree must be regenerated.

Similarly, when a template element or any of its descendants is mutated in any way, any bindings whose shadow tree was constructed from that element must be regenerated.

Regenerating a shadow tree consists of first destroying the existing shadow tree and then generating a new one.

4.1. Rules for Shadow Content Generation

If the binding element has no template element, then no shadow content will be generated for this binding.

The xml:base data of the cloned template element must be set so that the baseURI of nodes in the resulting shadow tree is the same as their pre-cloning counterparts. All shadow nodes' ownerDocument pointers are left pointing at their binding documents' Document node(s).

For bindings with implementations: the shadowTree member of the internal object must be set to be a reference to the template element clone (the root of the shadow tree).

The shadow tree is then applied to the bound element: the binding's shadow tree is placed in the appropriate place in the final flattened tree, explicit children are (re)distributed to the appropriate content elements, and the CSS cascade and inheritance is be computed along the new tree. (See: processing content elements.)

After this point, further bindings may need to be applied, or certain bindings may need to be removed (because of CSS inheritance or because the selectors that decide which elements match which bindings can be affected by the shadow tree being associated with the bound element).

Everything described in this section must be completed atomically — that is, the UA must not execute author scripts during this process.

Some implementations might optimize this algorithm, such as using "lazy evaluation" approaches and thereby postpone the cascade and inheritance operations.

4.2. Rules for Shadow Content Destruction

The destruction of a shadow tree consists of recreating the final flattened tree without the influence of that binding's shadow tree by redistributing the explicit children to the remaining shadow trees' content elements (or, if there are none, putting the nodes back directly under the bound element).

4.3. Attribute Forwarding

Attributes on shadow content elements can be tied to attributes on the bound element; then, whenever the attribute is set or removed on the bound element, the corresponding attribute on the shadow content is also set or removed. On any shadow content element, an attributes attribute can be used to specify a space-separated list of attributes that should be inherited. Attributes with namespaces can be defined using a namespace prefix and the attribute name separate by a colon.

Each entry in the attributes list can either simply list an attribute name (a string, such as value in the example above, containing no spaces, colons, asterisks, or hash marks), or it can specify an =-separated pair of attribute names (ditto), consisting of the attribute on the shadow content that should be tied to the attribute on the bound element. When two names are specified, the attribute to be added on the shadow content node is listed first, and the attribute of the bound element is second.

Each entry may also be suffixed by a single hash mark (#) followed by a type designation.

The attributes attribute's value must be parsed as follows. First, it must be split on spaces (treated as a space-separated value). Next, each resulting item must be matched against the attributes-item production in the following ABNF:

If any item does not match this pattern, then the item is in error and must be ignored. Other items in the list, if any, are not affected by this.

The values pseudo and attributes, if used such that they match the attribute production of the above ABNF, are in error and must be ignored.

4.3.1. Forwarding to and from text nodes

When specified on the left-hand side of the pair it indicates that the value of the attribute on the right-hand side are to be represented as text nodes underneath the shadow element in the final flattened tree. If the element has any child nodes in the DOM (any nodes, including comment nodes, whitespace text nodes, or even empty CDATA nodes) then the pair is in error and UAs must ignore it, meaning the attribute value is not forwarded. Otherwise, a text node must be created, and that text node will be placed under the element in the final flattened tree.

Text nodes created in this way are orphans; their parentNode, nextSibling, previousSibling, childNodes, firstChild, and lastChild attributes are all null or empty. Their ownerDocument attribute is set to the same as the shadow content node that generated them. (The only way one of these text nodes can be accessed is if the element is itself bound: the text node might then appear in a content element's xblChildNodes list.)

When used on the right-hand side, it indicates that any text nodes (including CDATA nodes and whitespace text nodes) that are explicit children of the bound element must have their data concatenated and the resulting value stored as the attribute on the left-hand side.

4.3.2. Forwarding language metadata

When used on the right-hand side, it indicates that the value to be copied is the natural language of the bound element, typically given by the attribute lang of that element or an ancestor, or by HTTP headers, or similar. If no language is defined, then the value to be copied must be the empty string.

The *lang value cannot occur by itself or on the left-hand side. If it does, it is in error and UAs must ignore that value in the element's attributes attribute.

4.3.3. Error handling

If an attribute is listed multiple times on the left hand side (or on its own), then the last designation wins (as if the attributes were each forwarded in turn, an earlier forwarding being overwritten by a later one), and all the ones before the first are in error.

4.3.4. Type specifiers

If the attribute or attribute pair is followed by a type designation, in the form of a hash mark character ("#") and by a type name, then the value must be processed as described for its type below before being forwarded.

4.3.5. Dynamic changes

The attributes attribute must be parsed when the binding is first applied and whenever the attribute's value changes. It must be applied (causing the relevant attributes and text nodes to be updated) when the shadow tree is generated, when the attribute is changed, and whenever any of the bound element's attributes or text nodes referred to by the attributes attribute change.

4.3.6. How Attribute Forwarding Affects the Shadow Tree

The element to which the attributes are forwarded (that is, the element with the attributes attribute specified) has its attributes mutated. Whenever attribute forwarding happens: existing attributes to which values are being forwarded must have their values changed, attributes that are being forwarded that do not yet exist on the shadow tree element must be added to the shadow tree element, attributes to which values would be forwarded but whose source attributes are not present on the bound element must be removed from the shadow tree element.

The shadow tree element's attributes can be changed dynamically, and this doesn't affect the attribute forwarding, until dynamic changes cause attribute forwarding to be performed again. When attribute forwarding is performed, all attributes are forwarded, even those that haven't changed on the bound element, thus blowing away any dynamic changes to the shadow tree element's attributes that are referenced by the attributes attribute.

4.4. Processing content Elements

A node is said to match a content element when the content element has no includes attribute, or when the element in question matches the selector given by that includes attribute.

If an element stops matching a content element that it is assigned to, then the bound element's explicit chlidren must be redistributed.

4.4.1. How Nodes are Distributed

XBL bindings can interleave shadow content between bound elements and their explicit children. They do so using XBL's content element. Any number of content nodes may be used in a binding's shadow content template.

In addition, the shadow trees of inherited bindings get inserted into the first inherited element in the binding.

The explicit children of an element are the nodes that are listed in the element's childNodes array, with the exception that any content elements in that array are instead replaced by whatever nodes they currently have assigned to them, or, if no nodes are assigned to that content element, by the child nodes of that content element. If an element's childNodes list is empty but the element has an attributes attribute that uses the *text value on the left hand side in a way that is not in error then its "explicit children" is the text node generated during attribute forwarding.

When the explicit children are distributed and assigned to the content elements in the bound element's shadow trees, the includes attribute determines which content element a given child is to be placed under.

If no includes attribute is specified, a content element is considered generic and will match on all content, including text nodes, CDATA nodes, comments, and so on.

If the includes attribute is specified, it must be interpreted as a selector, and only elements that match the selector apply to that content element. If the selector is invalid, the content element is in error and does not match any nodes. Matching of the elements to the selector is done without taking into account the shadow tree in which the content element itself is found. [SELECTORS]

Each node that is to be distributed (each explicit child node) must be assigned to a content element as follows:

4.4.2. When Nodes Are Redistributed

4.5. The Final Flattened Tree

The final flattened tree is the view of the document and shadow trees after XBL has been fully applied. It is only used for two things:

The final flattened tree must be constructed by taking the bound document's core DOM tree and performing the equivalent of the following steps on each bound element, until there are no more bound elements in the tree that have not been processed:

4.5.1. Terminology

Shadow content introduces the concept of shadow scope to nodes within a document. Because shadow content elements can also have bindings attached that generate their own shadow content, this scoping can be taken to an arbitrary level of nesting.

Shadow content nodes are in binding-level shadow scopes. Binding scopes are determined by the bound element to which the binding responsible for the generation of the shadow nodes is attached. The bound element itself is in the shadow scope of the content around it, and its binding's shadow content is in a deeper shadow scope. Shadow content that contains no elements that are themselves bound is said to be in the deepest, or innermost, shadow scope.

4.6. Handling DOM Changes

All of the nodes in the shadow tree are live. Whenever an element is inserted into, removed from, or appended to the DOM, and whenever its attributes or pseudo-class states are changed, all the children of bound elements must check that their assigned content element is still appropriate, following all the same rules that applied when first placing explicit children during shadow content generation. If one or more nodes stop fitting into any of the content elements then they no longer appear in the final flattened tree. Similarly, nodes that previously did not appear in the final flattened tree may start matching a content element and thus be inserted into the flattened tree.

It is possible to manipulate the shadow content contained underneath a bound element using standard DOM APIs. If shadow content that contains a content element is removed, then any explicit children assigned to that element are relocated to the first unlocked content elements that match them. If a content element's includes attribute is changed, then the explicit children of the binding's bound element must be redistributed appropriately.

content elements may be dynamically locked by manipulating their locked attribute. A locked content element cannot accept new children unless they are explicitly assigned to it using the setInsertionPoint() method. However, children already under a locked content element remain there while the element's includes attribute (or lack thereof) matches them.

Whenever the subtree of a template element in a binding document is dynamically modified, any shadow trees that were constructed by cloning that element must be regenerated.

4.7. Shadow Content and CSS

4.7.1. Selectors and Shadow Scopes

Bindings can interleave shadow elements between the bound element and its explicit children. (See: processing content elements.) In this situation, a new tree emerges that is different from the explicit content node tree. In addition to having a single explicit parent (the bound element) and a single set of children (the explicit children in the DOM tree), elements also have a set of shadow parents and shadow children (introduced by bindings when content elements were used). This necessarily affects the CSS model.

Combinators: CSS combinators, in the presence of XBL, must act as follows. This is intended to match the definitions of CSS in all cases other than when a selector would involve one or more XBL elements.

Pseudo-classes and pseudo-elements: Pseudo-classes and pseudo-elements are unchanged in the presence of XBL. They operate exclusively on the core DOM.

4.7.2. CSS Property Inheritance and Rendering

The final flattened tree determines how CSS properties (e.g., fonts and colors) are inherited. Elements must inherit from their parent node in the final flattened tree, regardless of what their DOM Core parent node is.

Similarly, the rendering is performed using the final flattened tree. Nodes that do not appear in the final flattened tree have no computed style (as if they were orphan nodes) and are not rendered.

4.7.3. The :bound-element Pseudo-Class

The :bound-element pseudo-class, when used from a binding, must match the bound element of that binding. If the selector is used somewhere other than in a binding's style sheet (i.e. with a style element in XBL) or in a content element's includes attribute, then it must match any bound element. [SELECTORS]

4.7.4. Matching Pseudo-Elements

Shadow nodes may be associated with various pre-defined pseudo-elements of the bound element. On any element in the shadow content template, an pseudo attribute (in the XBL namespace) can be used to specify the name of the pseudo to associate with that element.

If the pseudo-element name is not recognized, it is in error and the UA must ignore the attribute. User agents must not automatically recognize any pseudo-element (as this will break forwards-compatibility).

If an element has multiple nodes with the same pseudo-element, then they all match the relevant selector. Matching of nodes based on their pseudo-element is unaffected by the apply-author-sheets attribute.

These pseudo-element descriptions are purely advisory, and while authors are encouraged to use them for their predefined roles, it is valid to use them for other purposes.

In property descriptions, the term "all elements" in the "Applies To:" line includes these pseudo-elements, as they map directly to real elements in the binding.

User agents are required to support the above pseudo-element identifiers, in so far as they interact with XBL2. User agents may also support these same pseudo-elements for other purposes, e.g. as described in the CSS3 UI specification. [CSS3UI]

4.8. Shadow Content and xml:base

This section is intended to re-iterate what the xml:base specification already states, in case there is any question about how xml:base processing should work in shadow trees.

Relative xml:bases on nodes in shadow trees are resolved relative to their parentNode, or the ownerDocument if there is no parentNode.

4.9. Shadow Content and Other Things

4.9.1. General Rules

Shadow content is not considered part of a document, so elements that are defined to trigger when they are "inserted into the document" do not trigger during binding attachment.

IDs used in shadow content, as seen on XML Events nodes, in XHTML on the html:label element's for attribute, and in many other places, must be resolved in the context of the shadow scope and (failing that) the binding document, not the scope of the document into which the shadow content is inserted.

When an element's processing model is defined in terms of the element's child nodes or descendants, shadow trees do not affect the processing model (unless this is called out explicitly below). For instance, an HTML title element's behavior in determining the document title is unaffected by XBL, even if the title element is bound or has bound elements in its descendants.

When the nodes are cloned, their xml:base data remains as it was in the bindings document (see rules for content generation). Therefore URIs consisting of just fragment identifiers (such as those in url() notation in style attributes of, e.g., HTML nodes) refer to resources in the bindings document, not content in the bound document or the shadow tree.

This would cause trouble with attribute forwarding, so the attribute forwarding syntax allows attributes to be marked as being of type "url".

4.9.2. Style Blocks

The semantics of style elements (absent their scope attribute) is that they introduce new styles for their document. Since the document, in the case of anything in a shadow tree, is the binding document, that is the document that must be affected by such a style sheet.

Since the style sheets of such resource documents generally have no effect, placing style blocks in binding documents (without a scope attribute) is usually redundant. Such an element placed in a shadow content template does not affect the documents into which the shadow content is later inserted during binding attachment.

The style element is used with its scope attribute as a child of a template to provide styles for bindings, leveraging HTML's intrinsic semantics.

4.9.3. Script Blocks

Script elements, such as the HTML script element and its ilk, are typically evaluated only during parsing, or during parsing and when inserted into a document. In all cases, however, they are evaluated in the context of their owner document. Therefore such elements must only be evaluated during initial parsing, in the context of the binding document, and not during binding attachment.

4.9.4. HTML Forms

Forms and form controls in shadow trees don't interact with form controls and form elements in the bound document. Each document and shadow tree creates a new scope for forms and form controls.

4.9.5. SVG

Painting: When painting groups, for child elements that have shadow trees, instead of painting the child element itself, the group must paint the child nodes of the element's shadow tree's root template element.

Text: When rendering text, for descendant elements that have shadow trees, instead of using the element or its children directly, the user agent must use the child nodes of the element's shadow tree's root template element. (All other processing, e.g. handling of combining characters, must then be done as defined for SVG.)

ID references and URIs: When a URI identifies an element with a shadow tree, the SVG processor must use the first element node in the element's shadow tree's root template element's childNodes list instead of the element itself. If there are no elements, then the SVG document is in error. The SVG specification defines how to handle documents that are in error.

When a URI reference with a fragment identifier in a shadow tree references the binding document, then, if an elemnt in the shadow tree is identified by the fragment identifier, that is the element that must be used; otherwise, the fragment identifier must be matched against the actual binding document instead.

Animation elements: When an animation element would be implicitly associated with its parent element (e.g. when it has no xlink:href attribute), but that parent element is a template element that is the root of a shadow tree, then the animation element must instead be associated with the element found using the following algorithm:

4.10. Binding Style Sheets

Shadow content nodes and bound elements are styled using style sheets from a number of sources, depending on the values of certain attributes. When multiple bindings are applied to the same bound element, the sheets from each binding all contribute to the final set of style sheets to apply, the style sheets of the most derived binding being walked first. For each binding, the style sheets that apply are as follows, in the order given:

Scoped style sheets: A binding file can load style sheets using the style element. (See: loading style sheets.) These style sheets must be applied to the bound element and to all shadow content attached to the bound element.

If the binding was attached using CSS, the scoped style sheets have the same CSS origin as the sheet with the rule responsible for the binding. Style sheets used by bindings that are attached using the DOM or using <?xbl?> are treated as author-level sheets.

When bindings from multiple levels are applied to the same bound element, the style sheets that apply must cascade according to their own levels.

An element E is attached to binding U from the user agent style sheet, and binding A from the DOM, which places A in the author level. When the style sheets that apply to E are sorted, U must be applied at the UA level and A at the author level.

Author style sheets: While the apply-author-sheets attribute on the template element found at the root of the element's shadow tree is set to true, the rules specified in any author style sheets at outer shadow scopes (including those promoted to outer scopes using apply-binding-sheets, as defined below) must be applied to the shadow content. Otherwise, only those matched through predefined pseudo-elements are used, and other author-level sheets in higher shadow scopes must not be applied to the shadow content. (The bound element is always styled using the sheets of higher shadow scopes.)

By default, style sheets specified in bindings (as described above) are applied only to shadow content generated by bindings attached to the bound element and to the bound element itself. A second attribute, apply-binding-sheets, can be used to indicate that all descendants of the bound element, both shadow and explicit, can be styled by the sheets in the binding's document. This can be controlled on a per-insertion-point basis. While this attribute is set to true on a content node in the shadow tree DOM, any nodes that are assigned to that element, and any descendants of those nodes, must have the scoped style sheets of the binding (those that apply to the shadow content as described above) applied to them too, as if they had been promoted to the higher scope.

Sheets within each origin are always walked from the innermost shadow scope to the outermost shadow scope (with rules in the outermost shadow scope therefore overriding rules of equal specificity in the innermost shadow scope). With this ordering a binding that defines a widget can define a default look for the widget that can then be easily overridden by a client of the widget. For multiple bindings attached to the same element, the sheets are walked from the base binding to the most derived binding.

Since styles from both author style sheets and binding style sheets are applied to the bound element, it is possible for an infinite loop to form where an author sets the 'binding' property to a particular binding that then explicitly sets the 'binding' property to 'none' (or another binding). This specification does not take any precautions to avoid this, any more than it takes precautions to avoid loops caused by binding constructors explicitly calling removeBinding() to remove the binding itself and binding detachment event handlers reattaching the bindings. Similar potential loops exist also in underlying technologies, for example :hover rules that cause elements to no longer be hovered, or focus event handlers that move focus to an element and blur event handlers that move focus back to the element.

In so far as XBL is concerned, authors must avoid constructing such loops, and implementers must ensure that such loops do not prevent users from interacting with the user agent.

4.10.1. Summary of styling rules

The <style> element, with its scope attribute, is applied to the bound element and the shadow content that was generated by the binding. Its styles are also applied to explicit children (and their descendants) assigned to <content> elements whose apply-binding-sheets is set to true.

Continuing from the above, author sheets (styles from the bound document) are applied to the shadow content only if apply-author-sheets is set to true for the <template>.

Last, but not least, one can use author sheets to change the style of elements in the shadow content that use the pseudo attribute, as long as it matches them with pseudo-elements (irrespective of the apply-author-sheets setting).

5. Binding Implementations

Bindings can define methods and properties on a bound element using the implementation element. A binding implementation provides a new set of methods and properties that can be invoked from the bound element.

In general, each binding has an object that implements the XBLImplementation interface, along with any other interfaces that the implementation might implement. This is the implementation object for that instance of the binding. All elements implement the ElementXBL interface, whose xblImplementations member returns an object implementing XBLImplementationList. This object lists all the implementation objects for that bound element. (If the element is not a bound element, the list is empty.)

5.1. The XBLImplementation Interface

All implementation objects support the XBLImplementation interface (in addition to any other interfaces specific to the binding). By implementing the methods defined in this interface, bindings can be notified of the binding's state with respect to its environment.

The xblBindingAttached() method is called by the user agent after the binding has been attached. (See: binding attachment model.)

The xblLeftDocument() method is called by the user agent when the bound element, or one of its ancestors, or one of the elements in a higher shadow scope, is removed from the document. (See: handling insertion and removal from the document.)

If the implementation does not define one of these methods, then when that method is invoked, nothing must happen (as if all bindings had default implementations of those methods that were no-ops).

Authors should not start their own methods with the three letters "xbl". Future versions of this specification might add new callbacks to this interface, and if they do, those methods will start with the prefix "xbl".

5.2. The XBLImplementationList Interface

The xblImplementations attribute on all elements must return an instance of an XBLImplementationList object (the same object for the lifetime of the element), which is a live list of the implementation objects provided by the bindings for that bound element at any particular point in time.

The length attribute must return the number of implementation objects associated with the bound element, or zero if the element is not a bound element or has none of its bindings have implementations.

The item(n) method must return the nth implementation object associated with the bound element. If the index is not a number between zero and length-1 (inclusive), then the method must raise an INDEX_SIZE_ERR DOM exception. [DOM3CORE]

Objects that implement the XBLImplementationList interface must also have a [[Get]] method that, when invoked with a property name that is a number, acts like the item() method would when invoked with that argument.

5.3. Accessing Binding Implementations

Script can access binding implementations directly using the xblImplementations member. In addition, any attempts to access members of ElementXBL objects that do not correspond to methods or properties on the object itself but do correspond to members of one of the objects in the xblImplementations list must be forwarded to the last object in the xblImplementations list that is so matched.

5.4. Creation of Implementations

When a binding is attached, the user agent must follow the following steps to make the binding implementation available to scripts:

Conceptually, the internal and external objects together make the implementation object, but as far as the xblImplementations property's list is concerned, the external object is the one that is returned as the implementation object.

5.4.1. Compiling Bindings

When the user agent has to compile and run an XBL binding ECMAScript implementation, it must first obtain the script itself in the manner described in the section on loading and running scripts, and must then compile and execute the script using the binding document's global scope.

If the script evaluates to an object, then that is the implementation prototype object. Otherwise, there isn't one.

5.4.2. Invoking Methods on an Implementation Object

When function code of an implementation object is called, the user agent must set the this value to the internal object associated with the external object on which the function was invoked.

6. Event Handlers

6.1. Event Forwarding

Whenever an event passes through a bound element, whether during the capture, target, bubble, or default phases, the user agent must also invoke any appropriate event listeners attached to the binding's internal object.

When events are forwarded in this manner, the event handlers attached to the internal object must fire after any event handlers on the bound element itself in the capture phase, after the event has been retargeted to shadow nodes, if appropriate; and before any event handlers on the bound element itself in the target and bubble phases, before the event has been retargeted to the bound element, if appropriate. (See: event flow and targeting across shadow scopes.)

Event handlers must fire first on the most derived binding and then on its inherited binding, continuing all the way up the chains to the base binding. A derived handler then has a way of preventing the event from flowing to the handlers of the bindings it inherits from, by using the stopImmediatePropagation() method.

6.2. Event Flow and Targeting Across Shadow Scopes

DOM events can fire on shadow targets just as they can on explicit targets. Events must flow through the final flattened tree. As long as the event flows within the same shadow tree scope, it is no different from the behavior outlined in the DOM Events specification.

Whenever events originating from a shadow tree flow from a shadow element in that shadow tree to the bound element, one of two actions occurs. Either the event is retargeted so that the bound element becomes the target, or the event is stopped and flow proceeds to the next phase. Whenever an event is retargeted, the event is cloned, with the clone's target field set to the bound element.

The action taken (retarget vs. stop) is specific to the event type. In general, UI events must be retargeted and mutation events must be stopped. Exceptions to the rule are noted below. The goal of this retargeting or stopping is to stop outer shadow scopes from being exposed to nodes from inner shadow scopes, and to stop outer shadow scopes from getting apparently meaningless events that only make sense in the context of inner shadow scopes.

During the capture phase, the rules are exactly reversed. The first node to see the event is the node after which bubbling stops. The target node, when the event is passing through a node at a higher shadow scope than the event target, is always the bound element in whose shadow content the event target lies.

When an event is retargetted at a bound element, the bound element's event handlers must see the event only in the target phase. The capture phase listeners must not be triggered for the bound element.

The timing of event retargeting is such that when the event is forwarded to the internal object, the binding's handlers see the relevant shadow tree node as the target, rather than the bound element as the target. (See: event forwarding.)

Events bubble into deeper scopes; for example, an event fired on a bound element's explicit child bubbles into the element containing the content element the element was assigned to. This does not cause any event retargeting to take place, either when entering the deeper scope or when leaving it, since such an event does not actually originate in that shadow tree.

Any method invocations on any clones of the event object must also be forwarded to the original event and all the clones, so that attempts to stop propagation and cancel the default action affect the event regardless of how many scopes it has crossed.

6.3. The Default Phase

If an event bubbles through or is targeted at one or more bound elements, and the event is not canceled (after the capture, target, and bubble phases have all completed, its defaultPrevented attribute is still false), then the event's eventPhase attribute must be set to the value 0x78626C44 (2019716164), and then the event must be forwarded to the relevant internal objects of all the bound elements the event bubbled through or was targeted at in those bubble and target phases, in reverse tree order (starting from the target node and walking the tree towards the Document node), with currentTarget set to the relevant bound element each time. If the event is canceled (that is, if the defaultPrevented attribute becomes true) while being forwarded to one of these bound elements, subsequent bound elements must not receive the event.

If the event has a UA default action, it must only perform it if the defaultPrevented attribute is still false after it has been so forwarded.

The stopPropagation() and stopImmediatePropagation() methods must have no effect during this "default" phase.

6.4. The focus, DOMFocusIn, blur, and DOMFocusOut Events

If shadow content underneath a focusable bound element loses focus and shadow content also underneath the bound element takes focus, then both focus change events must be stopped. As far as the bound element is concerned, it retains focus throughout the two events. (Other specifications may go into more detail as to how to determine if an element can be focused.)

If the focus moves from the bound element's shadow content to a node completely outside the bound element, or vice versa, then the respective events must be retargetted instead.

The 'nav-index' property defined in the CSS UI module [CSS3UI] can be used to specify the tab order for focusable elements. This property can be specified on shadow content. Each shadow scope has a unique tab order. The 'nav-index' values used in one shadow scope are ignored by other shadow scopes. The tab order is resolved in the shadow tree first to produce a list of elements in the tab order. This list is substituted in place of the bound element in the bound element's tree tab order.

Because content in multiple shadow scopes can be focused, the CSS :focus pseudo-element is hierarchical in the presence of XBL, with up to one element in each shadow scope matching the pseudo-class. Style rules can be written with the assumption that they will match (in the above example) both the file control and the element focused inside the file control. In other words, an arbitrary chain of elements can be in the :focus state at the same time.

Further specifications may describe in more detail the interaction of arbitrary chains of elements that can be in the :focus state at the same time.

6.5. The mouseover and mouseout Events

Mouseover and mouseout events must be retargeted if the pointing device genuinely moves onto (enters) or is moved away (exits) the bound element (in addition to entering or exiting some shadow content). If, however, the user has simply moved the pointing device from one element in the shadow tree to another element in the same shadow tree, without entering or exiting the bound element itself, then the event must be stopped.

7. DOM Interfaces

7.1. The DocumentXBL Interface

The DocumentXBL interface contains methods for loading and obtaining binding documents. The interface is implemented by DOM documents that support having their elements bound by XBL.

7.2. The ElementXBL Interface

The ElementXBL interface contains methods for adding or removing bindings from an element. The interface is implemented by all Element nodes (regardless of whether they are currently involved with any XBL processing) and may be obtained using binding-specific casting methods on an Element interface.

7.2.1. Scoping and Access Using the DOM

In effect the shadow content exists in its own insulated pocket within the document, its shadow scope. Bound elements have no knowledge of their shadow children in terms of DOM Core [DOM3CORE]. The shadow content is not accessible via the childNodes list for the bound element, nor is it accessible using firstChild/nextSibling to iterate over the children of the bound element.

DOM methods that can be invoked on elements (e.g., getElementsByTagName()) will only see nodes that are in the same shadow scope. Methods invoked on the document (e.g., getElementById) only see nodes that are not in shadow trees.

On shadow content nodes, ownerDocument always points to the document from which the nodes were cloned.

Elements in different shadow scopes may have clashing IDs. IDs need only be unique within each shadow scope.

Elements that are the root of a shadow tree (the cloned template elements) cannot be inserted into a document. Any attempt to do so must raise a HIERARCHY_REQUEST_ERR.

Manipulating the DOM of a shadow tree (content elements being moved about or even removed altogether, attributes attributes being attached and modified, etc) must immediately cause the final flattened tree to be updated.

Because each bound element gets its own copy of the cloned template, changes to a bound element's shadow content only affect that bound element. Other bindings are unaffected.

If an element is added to the DOM dynamically, its shadow scope is that of its parent element. Adding an element as a child of a bound element causes that element to be assigned to an appropriate content element (if there is one — if there is not, the element does not appear anywhere in the final flattened tree).

7.3. The XBLContentElement Interface

7.4. The HTMLTemplateElement Interface

7.5. The HTMLBindingElement Interface

The Binding(selector) constructor must create a binding element. If there is an argument, then the element's element attribute must be set to that argument's value.

The getTemplate method must first check to see if the binding element has a template element child; if it does not, then one must be created and appended as a child of the element. Then, the method must return the first template element child of the binding element.

The setImplementation(implementationPrototypeObject) method must first check to see if the element has an implementation prototype object set. If it does (even if it is null), then the method must raise an INVALID_STATE_ERR exception. Otherwise, it must set the element's implementation prototype object's to implementationPrototypeObject.

The addToDocument() method must append the element to the first head element in the document, if any, or else the element's root element, if any, or else the Document object itself (as the root element).

7.6. The EventXBL Interface

Objects that implement the Event interface must also implement the EventXBL interface. [DOM3EVENTS]

The trusted attribute must return true if the user agent dispatched the event (e.g. in response to user action), and false otherwise (e.g. if an author script dispatched a synthetic event). Events fired by the user agent in response to untrusted events must themselves be untrusted.

8. Resources

8.1. Loading External Resources

8.1.1. Binding Documents

When the specification says that a binding document must be loaded unless it has already been loaded, then references to the same binding document (even if they are somewhat indirect, for example via HTTP redirects) must result in the same Document instance being reused, or shared.

To determine if two binding documents are the same, their final base URIs (after all redirects) are compared.

Such sharing of binding documents must be limited to binding documents loaded by a document, its binding documents, its scripts, and its style sheets. Nested documents and images do not share binding documents with each other or with their container document.

Binding documents that are currently loading count as binding documents that are already loaded for the purposes of this reuse mechanism.

References to binding documents that have another origin than the aspiring bound document must be treated as being in error, failing as if it was not available.

8.1.2. External Resources

When the specification simply says that the external resource must be loaded, without giving any caveats regarding multiple accesses of the same resource, then each reference must instantiate a new unique copy of the document.

For example, two style elements whose src attributes point to the same style sheet must create two different Stylesheet instances, such that mutating one does not affect the other.

Several XBL attributes are defined to contain URIs. All URIs may be relative. For relative URIs, the rules given in [XMLBASE] must be used to resolve the value to an absolute URI.

8.2. Loading and Running Scripts

Scripts in XBL may be found in implementation elements, or in resources that such elements point to.

For implementation elements, if a src attribute is present then the contents of the element must be ignored (even if fetching the specified URI fails).

If the content is inline, UAs must concatenate all the textual contents of text and CDATA child nodes, and must ignore any other, non-text nodes (such as elements and comments) along with all their children. All descendant elements must be processed, though, according to their semantics, before the XBL script block itself is executed.

If the content is not inline, then when the element is evaluated, the resource specified by the src attribute must be fetched. For implementation elements, while the external script is being fetched, the attachment of that binding must block. The contents of that file must be used directly, as specified by the ECMAScript language specification.

8.2.1. Scripting Model

Each document that runs script (including bound documents and binding documents) has a DocumentWindow object, a Window object, a global script scope, and a security context. In ECMAScript, the global script scope and the Window object are one and the same.

Script must always be executed in the context of the global script scope of the document specified by the script's element's ownerDocument DOM attribute. This implies that scripts from different bindings in the same binding document bound to different elements in the same bound document share the same scripting scope. If the bindings were defined in the document itself, then the scope is the same scope as for that document.

A binding document must inherit the security context of the document to which it is bound, not the security context of the domain from which it was fetched.

In binding documents, the location and history properties of the Window object, and the location and cookie properties of the DocumentWindow object, must return null, and any methods that are defined in terms of the browsing context's session history must do nothing. [HTML5]

8.3. Interpretation of URIs to XBL bindings

XBL attachment mechanisms use a URI to specify which binding to attach to the designated element.

This section defines how these URIs, which are used in the argument to the addBinding() method, and in the value of the 'binding' property, are to be interpreted.

The URI specifies a particular HTML or XML document. The user agent must fetch this resource (unless it has already been loaded).

If the URI contains a fragment identifier, then it must be processed as described in the relevant MIME type definition. The element targeted by the fragment identifier must be a binding element in the specified document, otherwise the URI is in error.

For example, if the binding document is sent as application/xhtml+xml, and the fragment identifier matches a binding element's id attribute, then that is the binding that is attached.

If there is no fragment identifier, the URI does not point to a correct binding and is in error.

When an attachment mechanism uses a URI that is in error (as per the last two paragraphs), then the user agent must act as if the attachment mechanism had not specified that binding.

Otherwise, the specified binding is attached to the element, as described for the relevant attachment mechanism.

9. Summaries of Elements, Attributes, and Events

9.1. Elements and Attributes

Element	Attributes	Content Model
`binding`	Global attributes `extends` `element`	`implementation` `template`
`implementation`	Global attributes `src`	Script
`template`	Global attributes `apply-author-sheets` `allow-selectors-through`	`content` `inherited` Non-XBL elements, optionally with `attributes` and `pseudo` attributes
`content`	Global attributes `includes` `apply-author-sheets` `locked`	`inherited` Non-XBL elements, optionally with `attributes` and `pseudo` attributes
`inherited`	Global attributes	`content` Non-XBL elements, optionally with `attributes` and `pseudo` attributes

9.2. Events

Event Name	Interface	Target when fired by UA	Bubbles?	Cancelable?	Default Action
`xbl-bound`	`Event`	Bound element	✓ Bubbles	—	None
`xbl-bindings-are-ready`	`Event`	Bound document's root element	✓ Bubbles	—	None

9.3. Implementations

Acknowledgments

David Hyatt developed XBL 1.0 and provided guidance for the development of XBL 2.0.

The editor would like to thank Alex Danilo, Alex Russell, Alex Vincent, Anne van Kesteren, Axel Hecht, Antoine Quint, Benjamin Smedberg, Bjoern Hoehrmann, Boris Zbarsky, Brendan Eich, Cameron McCormack, Chris Lilley, Christophe Jolif, Cyril Concolato, Darryl Fuller, David Håsäther, Dean Jackson, Dimitri Glazkov, Jon Ferraiolo, Jonas Sicking, Karl Dubost, L. David Baron, Lachlan Hunt, Liam Quin, Maciej Stachowiak, Marcos Caceres, Mark Baker, Micah Dubinko, Mihai Sucan, Mikko Pohja, Mohamed Zergaoui, Norman Walsh, Peter Sorotokin, Robin Berjon, Ruud Steltenpool, Sam Weinig, Sean Hogan, Simon Pieters, Steve K. Speicher, Steve Zilles, Tab Atkins, Tim Rowley, and Tom Pike for their contributions to this specification.

XBL 2.0

W3C Working Group Note 24 May 2012

Abstract

Status of This Document

Table of Contents

1. Introduction

1.1. Editor's note

1.2. Relationship to XBL1

1.3. Relationship to XSLT

1.4. Terminology and Conventions

1.5. Conformance

1.5.1. Error Handling

1.5.2. Attributes Containing Selectors

1.5.3. Attributes Containing Space-Separated Values

1.5.4. Attributes Containing URIs

1.5.5. Attributes Containing Keywords

1.6. Security Concerns

2. XBL Elements

2.1. The binding Element

Attributes

2.2. The implementation Element

Attributes

2.3. The template Element

Attributes

2.4. The content Element

Attributes

2.5. The inherited Element

Attributes

2.6. The attributes Attribute

2.7. The pseudo Attribute

3. Binding Attachment and Detachment

3.1. The Bindings-Are-Ready State

3.2. Attachment using <binding element="">

3.2.1. Importing Binding Documents

3.3. Attachment using CSS

3.3.1. The 'binding' Property

3.3.2. Processing Model

3.3.3. Examples

3.4. Attachment using the DOM

3.5. Binding Attachment Model

3.6. Handling Insertion and Removal from the Document

3.7. Binding Inheritance

3.7.1. Explicit Inheritance

3.7.2. Implicit Inheritance

3.7.3. Mixing Implicit and Explicit Inheritance

3.8. Attachment During Document Load

3.9. Binding Detachment Model

4. Shadow Content

4.1. Rules for Shadow Content Generation

4.2. Rules for Shadow Content Destruction

4.3. Attribute Forwarding

4.3.1. Forwarding to and from text nodes

4.3.2. Forwarding language metadata

4.3.3. Error handling

4.3.4. Type specifiers

4.3.5. Dynamic changes

4.3.6. How Attribute Forwarding Affects the Shadow Tree

4.4. Processing content Elements

4.4.1. How Nodes are Distributed

4.4.2. When Nodes Are Redistributed

4.5. The Final Flattened Tree

4.5.1. Terminology

4.6. Handling DOM Changes

4.7. Shadow Content and CSS

4.7.1. Selectors and Shadow Scopes

4.7.2. CSS Property Inheritance and Rendering

4.7.3. The :bound-element Pseudo-Class

4.7.4. Matching Pseudo-Elements

4.8. Shadow Content and xml:base

4.9. Shadow Content and Other Things

4.9.1. General Rules

4.9.2. Style Blocks

4.9.3. Script Blocks

4.9.4. HTML Forms

4.9.5. SVG

4.10. Binding Style Sheets

4.10.1. Summary of styling rules

5. Binding Implementations

5.1. The XBLImplementation Interface

5.2. The XBLImplementationList Interface

2.1. The `binding` Element

2.2. The `implementation` Element

2.3. The `template` Element

2.4. The `content` Element

2.5. The `inherited` Element

2.6. The `attributes` Attribute

2.7. The `pseudo` Attribute

3.2. Attachment using `<binding element="">`

3.3.1. The '`binding`' Property

4.4. Processing `content` Elements

4.7.3. The `:bound-element` Pseudo-Class

4.8. Shadow Content and `xml:base`

5.1. The `XBLImplementation` Interface

5.2. The `XBLImplementationList` Interface

6.4. The `focus`, `DOMFocusIn`, `blur`, and `DOMFocusOut` Events

6.5. The `mouseover` and `mouseout` Events

7.1. The `DocumentXBL` Interface

7.2. The `ElementXBL` Interface

7.3. The `XBLContentElement` Interface

7.4. The `HTMLTemplateElement` Interface

7.5. The `HTMLBindingElement` Interface

7.6. The `EventXBL` Interface