Polyglot Markup: HTML-Compatible XHTML Documents

W3C Working Draft 19 October 2010

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Eliot Graff, Microsoft Corporation


A document that uses polyglot markup is document that is a stream of bytes that parses into identical document trees (with the exception of the xmlns attribute on the root element) when processed as HTML and when processed as XML. Polyglot markup that meets a well defined set of constraints is interpreted as compatible, regardless of whether they are processed as HTML or as XHTML, per the HTML5 specification. Polyglot markup uses a specific DOCTYPE, namespace declarations, and a specific case—normally lower case but occasionally camel case—for element and attribute names. Polyglot markup uses lower case for certain attribute values. Further constraints include those on empty elements, named entity references, and the use of scripts and style.

Status of This Document

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 summarizes design guidelines for authors who wish their XHTML or HTML documents to validate on either HTML or XML parsers, assuming the parsers to be HTML5-compliant. This specification is intended to be used by web authors. It is not a specification for user agents and creates no obligations on user agents. Note that this recommendation does not define how HTML5-conforming user agents should process HTML documents. Nor does it define the meaning of the Internet Media Type text/html. For user agent guidance and for these definitions, see [HTML5] and [RFC2854].

This document was published by the HTML working group as a Working Draft. This document is intended to become a W3C Recommendation. Please submit comments regarding this document by using the W3C's public bug database ( http://www.w3.org/Bugs/Public/) with the product set to HTML WG and the component set to HTML/XHTML Compatibility Authoring Guide (ed: Eliot Graff). If you cannot access the bug database, submit comments to public-html@w3.org@w3.org (subscribe, archives) and arrangements will be made to transpose the comments to the bug database. All feedback is welcome.

Publication as a Working Draft 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.

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.

Table of Contents

1. Introduction

This section is non-normative.

It is often valuable to be able to serve HTML5 documents that are also valid XML documents. An author may, for example, use XML tools to generate a document, and they and others may process the document using XML tools. These documents are served as text/html. The language used to create documents that can be parsed by both HTML and XML parsers is called polyglot markup. Polyglot markup is the overlap language of documents which are both HTML5 documents and XML documents.

2. Processing Instructions and the XML Declaration

Processing Instructions and the XML Declaration are both forbidden in polyglot markup.

3. Specifying a Document's Character Encoding

Polyglot markup uses either UTF-8 or UTF-16. UTF-8 is preferred. When polyglot markup uses UTF-16, it must not include a BOM. When polyglot markup uses UTF-16, it must include the BOM indicating little-endian UTF-16 or big-endian UTF-16.

Polyglot markup declares character encoding one of two ways:

Using <meta charset="*"/> has no effect in XML. Therefore, polyglot markup may use <meta charset="*"/> in combination with BOM, as long the meta element specifies the same character encoding as the BOM. In addition, the meta tag may be used in the absence of a BOM as long as it matches the already specified encoding. Note that the W3C Internationalization (i18n) Group recommends to always include a visible encoding declaration in a document, because the declaration helps developers, testers, or translation production managers to check the encoding of a document visually.


Polyglot markup must have a document type declaration (DOCTYPE) specified by section 8.1.1 of [HTML5]. In addition, the DOCTYPE must conform to the following rules:

Note that polyglot markup cannot use document type declarations for HTML4, HTML3, or HTML2, regardless of whether they contain a URI or not and regardless of their effect in HTML5 parsers, as these document type declarations are not compatible with XHTML.

5. Namespaces

The following rules apply to namespaces used in polyglot markup.

5.1 Element-Level Namespaces

[HTML5] introduces undeclared (native) default namespaces for the root HTML element <html>, the root SVG element <svg>, and the root MathML element <math>. The following default namespaces must be declared in polyglot markup, to maintain XML-compatibility [XML10]:

Polyglot markup must not declare any other default or prefixed element namespace, because [HTML5] does not natively support the declaring of any other default or prefixed element namespace.

5.2 Attribute-Level Namespaces

[HTML5] introduces undeclared (native) support for attributes in the XLink namespace and with the prefix xlink:. Polyglot markup must declare the XLink namespace on the HTML root element (<html>) or once on the foreign element where is used (<svg> or <mathml>), to maintain XML-compatibility [XML10].

In polyglot markup, the xlink prefix uses the namespace declaration xmlns:xlink="http://www.w3.org/1999/xlink" before using the xlink prefix for the following elements:

Furthermore, polyglot markup defines the xlink prefix only on foreign elements (any SVG or MathML element) but not the root <html> element or any other HTML element.

Note that there are other prefixed attributes that can be used beyond xlink:href (such as xml:base). Polyglot markup does not declare these prefixes via xmlns. The prefixes are implicitly declared in XML and are automatically applied to the appropriate attributes in HTML.

6. Elements

Polyglot markup conforms to the following rules regarding elements.

6.1 Required Elements

Polyglot markup must explicitly have a tbody element surrounding groups of tr elements within a table element. HTML parsers insert the tbody element, but XML parsers do not, thus creating different DOMs.



Polyglot markup must explicitly have a colgroup element surrounding groups of col elements within a table element. HTML parsers insert the colgroup element, but XML parsers do not, thus creating different DOMs.



6.2 Elements that Cannot Be Used in Polyglot Markup

Polyglot markup does not use the <noscript> element, because the <noscript> element must not be used in XML documents. [HTML5]

6.3 Case-Sensitivity

The following guidelines apply to any usage of element names, attribute names, or attribute values in markup, script, or CSS. Polyglot markup uses lower case letters for all ASCII letters. For non-ASCII letters—such as Greek, Cyrillic, or non-ASCII Latin letters—polyglot markup respects case sensitivity as it is called for.

6.3.1 Element Names

Polyglot markup uses the correct case for element names.

  • Polyglot markup uses lowercase letters for all HTML element names.
  • Polyglot markup uses lowercase letters for all MathML element names.
  • Polyglot markup uses lowercase letters for all SVG element names except the following, which must be in mixed case:
    • altGlyph
    • altGlyphDef
    • altGlyphItem
    • animateColor
    • animateMotion
    • animateTransform
    • clipPath
    • feBlend
    • feColorMatrix
    • feComponentTransfer
    • feComposite
    • feConvolveMatrix
    • feDiffuseLighting
    • feDisplacementMap
    • feDistantLight
    • feFlood
    • feFuncA
    • feFuncB
    • feFuncG
    • feFuncR
    • feGaussianBlur
    • feImage
    • feMerge
    • feMergeNode
    • feMorphology
    • feOffset
    • fePointLight
    • feSpecularLighting
    • feSpotLight
    • feTile
    • feTurbulence
    • foreignObject
    • glyphRef
    • linearGradient
    • radialGradient
    • textPath

6.3.2 Attribute Names

Polyglot markup uses the correct case for attribute names.

  • Polyglot markup uses lowercase letters in attribute names for all HTML elements.
  • Polyglot markup uses lowercase letters in attribute names for all MathML elements except the lowercase definitionurl must be changed to the mixed case definitionURL.
  • Polyglot markup uses lowercase letters in attribute names for all SVG elements except the following, which must be in mixed case:
    • attributeName
    • attributeType
    • baseFrequency
    • baseProfile
    • calcMode
    • clipPathUnits
    • contentScriptType
    • contentStyleType
    • diffuseConstant
    • edgeMode
    • externalResourcesRequired
    • filterRes
    • filterUnits
    • glyphRef
    • gradientTransform
    • gradientUnits
    • kernelMatrix
    • kernelUnitLength
    • keyPoints
    • keySplines
    • keyTimes
    • lengthAdjust
    • limitingConeAngle
    • markerHeight
    • markerUnits
    • markerWidth
    • maskContentUnits
    • maskUnits
    • numOctaves
    • pathLength
    • patternContentUnits
    • patternTransform
    • patternUnits
    • pointsAtX
    • pointsAtY
    • pointsAtZ
    • preserveAlpha
    • preserveAspectRatio
    • primitiveUnits
    • refX
    • refY
    • repeatCount
    • repeatDur
    • requiredExtensions
    • requiredFeatures
    • specularConstant
    • specularExponent
    • spreadMethod
    • startOffset
    • stdDeviation
    • stitchTiles
    • surfaceScale
    • systemLanguage
    • tableValues
    • targetX
    • targetY
    • textLength
    • viewBox
    • viewTarget
    • xChannelSelector
    • yChannelSelector
    • zoomAndPan

6.3.3 Attribute Values

Polyglot markup uses lowercase letters for the values of the attributes in the following list when they exist on HTML elements. More specifically, where required, polyglot markup must use lower case letters for all ASCII letters in these attribute values; however, polyglot markup respects case sensitivity for non-ASCII letters such as Greek, Cyrillic, or non-ASCII Latin letters. For attribute values on HTML elements other than those in the following list, polyglot markup may use mixed case letters.

Because XML is case sensitive, polyglot markup also requires case to be consistent for values between markup, DOM APIs, and CSS. In addition, polyglot markup respects the case sensitivity of all other attribute values. Although polyglot markup must always have lowercase values of the attributes in the following list when they exist on HTML elements, attributes not in this list and attributes on non-HTML elements may have values made of mixed case letters. Note that other specifications, such as RDFa, may place additional restrictions on the allowed values of certain attributes.

  • accept
  • accept-charset
  • align
  • alink
  • axis
  • bgcolor
  • charset
  • checked
  • clear
  • codetype
  • color
  • compact
  • declare
  • defer
  • dir
  • direction
  • disabled
  • enctype
  • face
  • frame
  • hreflang
  • http-equiv
  • lang
  • language
  • link
  • media
  • method
  • multiple
  • nohref
  • noresize
  • noshade
  • nowrap
  • readonly
  • rel
  • rev
  • rules
  • scope
  • scrolling
  • selected
  • shape
  • target
  • text
  • type
  • valign
  • valuetype
  • vlink

6.4 Empty Elements

Polyglot markup uses only the elements in the following list as empty elements.

Polyglot markup uses the minimized tag syntax for empty elements, e.g. <br/>. The alternative syntax <br></br> allowed by XML gives uncertain results in many existing user agents.

Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) polyglot markup does not use the minimized form (e.g. the document uses <p></p> and not <p />).

Note that MathML and SVG elements may be either self-closing or contain content.

6.5 Elements with Special Considerations

The following elements or their considerations require exceptions to the general rules for polyglot markup.

6.5.1 White Space in <textarea> and <pre> Elements

Within polyglot markup, the text within either a <textarea> or <pre> element must not begin with white space.

7. Attributes

Because of attribute-value normalization in XML [XML10], polyglot markup does not contain tabs, line feeds, or carriage returns within CDATA attributes.

Polyglot markup surrounds all attribute values with quotation marks. Attribute values may be surrounded either by single quotation marks or by double quotation marks.

See also Attribute Values.

7.1 Disallowed Attributes

The following attributes are not allowed in polyglot markup. These attributes have effects in documents parsed as XML but do not have effects in documents parsed as text/html. The HTML5 spec therefore defines them as invalid in text/html documents. [HTML5]

Note that the xml:space and xml:base attributes are allowed on SVG and MathML elements.

7.2 Language Attributes

When using language attributes, polyglot markup must use both the lang and xml:lang attributes. Neither attribute is to be used without the other, and the values for both lang and xml:lang must be the same.

Polyglot markup should use the language attributes in the html element to set the default language for the document.

8. Named Entity References

Polyglot markup uses only the following named entity references:

For entities beyond the previous list, a polyglot document uses character references. For example, polyglot markup uses &#xA0; instead of &nbsp;. Note that polyglot markup may use decimal values for escape characters (such as &#160; in the previous example); however, the Character Model for the World Wide Web recommends that content should use the hexadecimal form of character escapes rather than the decimal form when there are both. [CHARMOD]

9. Script and Style

Script and style commands should be included by linking to external files rather than including them in-line. However, polyglot markup must not link to an external stylesheet by using the xml-stylesheet processing instruction. See also Processing Instructions and the XML Declaration.

The following examples show the proper way to include external script and style, respectively:

<script src="external.js"></script>
<link rel="stylesheet" href="external.css"/>

Although document.write() and document.writeln() are valid in an HTML document, neither function may be used in XHTML. Therefore, neither is used in polyglot markup. Instead, use the innerHTML property for both HTML and XHTML. Note that the innerHTML property takes a string. XML parsers parse the string as XML in XHTML. HTML parsers parse the string as HTML in HTML. Because of the difference in parsing, if you send the parser content that does not follow the rules for polyglot markup the results will differ for a DOM create with an XML parser and one created with an HTML parser.

9.1 External Script and Style

Polyglot markup uses external scripts if that document's script or style sheet uses < or & or ]]> or --. Note that XML parsers are permitted to silently remove the contents of comments; therefore, the historical practice of hiding scripts and style sheets within comments to make the documents backward compatible is likely to not work as expected in XML-based user agents.

9.2 In-line Script and Style

If polyglot markup must use script or style commands within its source code, either use safe content or wrap the command in a CDATA section. However, polyglot markup does not use a CDATA section unless it is being used within foreign content.

9.2.1 Safe Content

Safe content is content that does not contain a < or & character. The following example is safe because it does not contain problematic characters within the <script> tag.


9.2.2 Wrapping a Command in a CDATA Section

Note that you cannot achieve same DOM in both XHTML and HTML by using in-line commands in a CDATA section. However, this is not usally a problem unless the code has a dependency on the exact number of text nodes under a <script> or <style> element. The following examples show in-line script and style commands wrapped in a CDATA section.

	(script goes here)
	(styles go here)

When using MathML or SVG, the parser follows the XML parsing rules. Polyglot markup does not rely on getting a CDATA instance from the DOM when using MathML or SVG, because the HTML parser does not create a CDATA instance in the DOM.

10. Exceptions from the Foreign Content Parsing Rules

A. Acknowledgements

Many thanks to Daniel Glazman, Richard Ishida, Tony Ross, Sam Ruby, Jonas Sicking, Leif Halvard Silli, Henri Sivonen, Manu Sporny, and Philip Taylor. Special thanks to the W3C TAG and the W3C Internationalization (i18n) Core Working Group.

B. References

B.1 Normative references

Martin J. Dürst; et al. Character Model for the World Wide Web 1.0: Fundamentals. 15 February 2005. W3C Recommendation. URL: http://www.w3.org/TR/2005/REC-charmod-20050215/
Ian Hickson; David Hyatt. HTML 5. 4 March 2010. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2010/WD-html5-20100304/
R. Fielding; et al. Hypertext Transfer Protocol - HTTP/1.1. June 1999. Internet RFC 2616. URL: http://www.ietf.org/rfc/rfc2616.txt
D. Connolly; L. Masinter. The 'text/html' Media Type. June 2000. Internet RFC 2854. URL: http://www.rfc-editor.org/rfc/rfc2854.txt
C. M. Sperberg-McQueen; et al. Extensible Markup Language (XML) 1.0 (Fifth Edition). 26 November 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-xml-20081126/

B.2 Informative references

No informative references.