HTML 5.1

When the element is created, if the attribute is absent, the additional information should be hidden; if the attribute is present, that information should be shown. Subsequently, if the attribute is removed, then the information should be hidden; if the attribute is added, the information should be shown.

The user agent should allow the user to request that the additional information be shown or hidden. To honor a request for the details to be shown, the user agent must set the open attribute on the element to the value open. To honor a request for the information to be hidden, the user agent must remove the open attribute from the element.

Whenever the open attribute is added to or removed from a details element, the user agent must queue a task that runs the following steps, which are known as the details notification task steps, for this details element:

1. If another task has been queued to run the details notification task steps for this details element, then abort these steps.

   When the open attribute is toggled several times in succession, these steps essentially get coalesced so that only one event is fired.

2. Fire a simple event named toggle at the details element.

The task source for this task must be the DOM manipulation task source.

The open IDL attribute must reflect the open content attribute.

<section class="progress window">
 <h1>Copying "Really Achieving Your Childhood Dreams"</h1>
 <details>
  <summary>Copying... <progress max="375505392" value="97543282"></progress> 25%</summary>
  <dl>
   <dt>Transfer rate:</dt> <dd>452KB/s</dd>
   <dt>Local filename:</dt> <dd>/home/rpausch/raycd.m4v</dd>
   <dt>Remote filename:</dt> <dd>/var/www/lectures/raycd.m4v</dd>
   <dt>Duration:</dt> <dd>01:16:27</dd>
   <dt>Color profile:</dt> <dd>SD (6-1-6)</dd>
   <dt>Dimensions:</dt> <dd>320×240</dd>
  </dl>
 </details>
</section>

<details>
 <summary><label for=fn>Name & Extension:</label></summary>
 <p><input type=text id=fn name=fn value="Pillar Magazine.pdf">
 <p><label><input type=checkbox name=ext checked> Hide extension</label>
</details>

<style>
 details > summary { transition: color 1s; color: black; }
 details[open] > summary { color: red; }
</style>
<details>
 <summary>Automated Status: Operational</summary>
 <p>Velocity: 12m/s</p>
 <p>Direction: North</p>
</details>

---

A menu is a currently relevant menu element if it is the child of a currently relevant menu element, or if it is the designated pop-up menu of a button element that is not inert, does not have a hidden attribute, and is not the descendant of an element with a hidden attribute.

---

A pop-up menu consists of a list of zero or more menu items, which can be any of:

• Commands, which can be marked as default commands (menuitem)
• Separators (hr)
• Other menus, which allows the list to be nested (menu)

To construct and show a menu for a particular menu element and with a particular element as a subject, the user agent must run the following steps:

1. Let the menu be an empty list of the type described above.

2. Run the menu builder steps for the menu element using the menu prepared in the previous list as the output.

   The menu builder steps for a menu element using a specific menu as output are as follows: For each child node of the menu in tree order, run the appropriate steps from the following list:

   If the child is a menuitem element that defines a command

   Append the command to the menu, respecting its facets. If the menuitem element has a default attribute, mark the command as being a default command.

   If the child is an hr element

   Append a separator to the menu.

   If the child is a menu element with no label attribute

   Append a separator to the menu, then run the menu builder steps using this child menu element for the same menu, then append another separator to the menu.

   If the child is a menu element with a label attribute

   Create a new submenu as an empty list of the type described above, and construct it by running the menu builder steps for the child menu element using the new submenu as the output. Then, append the submenu to the menu, using the value of the child menu element's label attribute as the label of the submenu.

   Otherwise

   Ignore the child node.

3. Remove any submenu with no label, or whose label is the empty string, in the menu or any submenus.

4. Remove any menu item with no label, or whose label is the empty string, in the menu or any submenus.

5. Collapse all sequences of two or more adjacent separators in the menu or any submenus to a single separator.

6. Remove all separators at the start or end of the menu and any submenus.

7. Display the menu to the user, and let the algorithm that invoked this one continue.

   If the user selects a menu item that corresponds to an element that still represents a command when the user selects it, then the UA must invoke that command's Action. If the command's Action is defined as firing a click event, either directly or via the run synthetic click activation steps algorithm, then the relatedTarget attribute of that click event must be initialized to the subject passed to this construct and show a menu algorithm.

   Pop-up menus must not, while being shown, reflect changes in the DOM. The menu is constructed from the DOM before being shown, and is then immutable.

---

The type IDL attribute must reflect the content attribute of the same name, limited to only known values.

The label IDL attribute must reflect the content attribute of the same name.

<menu> 
 <li>
  <button type=menu value="File" menu="filemenu">
  <menu id="filemenu" type="popup">
   <menuitem onclick="fnew()" label="New...">
   <menuitem onclick="fopen()" label="Open...">
   <menuitem onclick="fsave()" label="Save">
   <menuitem onclick="fsaveas()" label="Save as...">
  </menu>
 </li>
 <li>
  <button type=menu value="Edit" menu="editmenu">
  <menu id="editmenu" type="popup">
   <menuitem onclick="ecopy()" label="Copy">
   <menuitem onclick="ecut()" label="Cut">
   <menuitem onclick="epaste()" label="Paste">
  </menu>
 </li>
 <li>
  <button type=menu value="Help" menu="helpmenu">
  <menu id="helpmenu" type="popup">
   <menuitem onclick="location='help.html'" label="Help">
   <menuitem onclick="location='about.html'" label="About">
  </menu>
 </li>
</menu>

<form action="dosearch.pl">
 <p><label>Enter search terms: <input type="text" name="terms"></label></p>
 <p><input type=submit contextmenu=formmenu id="submitbutton"></p>
 <p hidden><input type=reset id="resetbutton"></p>
 <menu type=popup id=formmenu>
  <menuitem command="submitbutton" default>
  <menuitem command="resetbutton">
 </menu>
</form>

---

The type IDL attribute must reflect the content attribute of the same name, limited to only known values.

The label, icon, disabled, checked, and radiogroup, and default IDL attributes must reflect the respective content attributes of the same name.

The command IDL attribute must return the master command, if any, or null otherwise.

---

If the element's Disabled State is false (enabled) then the element's activation behavior depends on the element's type and command attributes, as follows:

If the element has a master command set by its command attribute

The user agent must run synthetic click activation steps on the element's master command.

If the type attribute is in the Checkbox state

If the element has a checked attribute, the UA must remove that attribute. Otherwise, the UA must add a checked attribute, with the literal value checked.

If the type attribute is in the Radio state

If the element has a parent, then the UA must walk the list of child nodes of that parent element, and for each node that is a menuitem element, if that element has a radiogroup attribute whose value exactly matches the current element's (treating missing radiogroup attributes as if they were the empty string), and has a checked attribute, must remove that attribute.

Then, the element's checked attribute must be set to the literal value checked.

Otherwise

The element's activation behavior is to do nothing.

Firing a synthetic click event at the element does not cause any of the actions described above to happen.

If the element's Disabled State is true (disabled) then the element has no activation behavior.

<button type=menu menu=editmenu>Commands...</button>
<menu type="popup" id="editmenu">
 <menuitem type="radio" radiogroup="alignment" checked="checked"
          label="Left" icon="icons/alL.png" onclick="setAlign('left')">
 <menuitem type="radio" radiogroup="alignment"
          label="Center" icon="icons/alC.png" onclick="setAlign('center')">
 <menuitem type="radio" radiogroup="alignment"
          label="Right" icon="icons/alR.png" onclick="setAlign('right')">
 <hr>
 <menuitem type="command" disabled
          label="Publish" icon="icons/pub.png" onclick="publish()">
</menu>

<form name="npc">
 <label>Character name: <input name=char type=text contextmenu=namemenu required></label>
 <menu type=popup id=namemenu>
  <menuitem label="Pick random name" onclick="document.forms.npc.elements.char.value = getRandomName()">
  <menuitem label="Prefill other fields based on name" onclick="prefillFields(document.forms.npc.elements.char.value)">
 </menu>
</form>

4.11.5.2 Processing model

Each element has an assigned context menu, which can be null. If an element A has a contextmenu attribute, and there is an element with the ID given by A's contextmenu attribute's value in A's home subtree, and the first such element in tree order is a menu element whose type attribute is in the popup menu state, then A's assigned context menu is that element. Otherwise, if A has a parent element, then A's assigned context menu is the assigned context menu of its parent element. Otherwise, A's assigned context menu is null.

When an element's context menu is requested (e.g. by the user right-clicking the element, or pressing a context menu key), the user agent must apply the appropriate rules from the following list:

If the user requested a context menu using a pointing device

The user agent must fire a trusted event with the name contextmenu, that bubbles and is cancelable, and that uses the MouseEvent interface, at the element for which the menu was requested. The context information of the event must be initialized to the same values as the last MouseEvent user interaction event that was fired as part of the gesture that was interpreted as a request for the context menu.

Otherwise

The user agent must fire a synthetic mouse event named contextmenu that bubbles and is cancelable at the element for which the menu was requested.

Typically, therefore, the firing of the contextmenu event will be the default action of a mouseup or keyup event. The exact sequence of events is UA-dependent, as it will vary based on platform conventions.

The default action of the contextmenu event depends on whether or not the element for which the menu was requested has a non-null assigned context menu when the event dispatch has completed, as follows.

If the assigned context menu of the element for which the menu was requested is null, the default action must be for the user agent to show its default context menu, if it has one.

Otherwise, let subject be the element for which the menu was requested, and let menu be the assigned context menu of target immediately after the contextmenu event's dispatch has completed. The user agent must fire a trusted event with the name show at menu, using the RelatedEvent interface, with the relatedTarget attribute initialized to subject. The event must be cancelable.

If this event (the show event) is not canceled, then the user agent must construct and show the menu for menu with subject as the subject.

The user agent may also provide access to its default context menu, if any, with the context menu shown. For example, it could merge the menu items from the two menus together, or provide the page's context menu as a submenu of the default menu. In general, user agents are encouraged to de-emphasize their own contextual menu items, so as to give the author's context menu the appearance of legitimacy — to allow documents to feel like "applications" rather than "mere Web pages".

User agents may provide means for bypassing the context menu processing model, ensuring that the user can always access the UA's default context menus. For example, the user agent could handle right-clicks that have the Shift key depressed in such a way that it does not fire the contextmenu event and instead always shows the default context menu.

---

The contextMenu IDL attribute must reflect the contextmenu content attribute.

<img src="cats.jpeg" alt="Cats" contextmenu=catsmenu>
<menu type="popup" id="catsmenu">
 <menuitem label="Pet the kittens" onclick="kittens.pet()">
 <menuitem label="Cuddle with the kittens" onclick="kittens.cuddle()">
 <menu label="Feed the kittens">
  <menuitem label="Fish" onclick="kittens.feed(fish)">
  <menuitem label="Chicken" onclick="kittens.feed(chicken)">
 </menu>
</menu>

[Constructor(DOMString type, optional RelatedEventInit eventInitDict)]
interface RelatedEvent : Event {
  readonly attribute EventTarget? relatedTarget;
};

dictionary RelatedEventInit : EventInit {
  EventTarget? relatedTarget;
};

The relatedTarget attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents the other event target that is related to the event.

The commandType attribute must return a string whose value is either "command", "radio", or "checkbox", depending on whether the Type of the command defined by the element is "command", "radio", or "checkbox" respectively. If the element does not define a command, it must return null.

The commandLabel attribute must return the command's Label, or null if the element does not define a command or does not specify a Label.

The commandIcon attribute must return the absolute URL of the command's Icon. If the element does not specify an icon, or if the element does not define a command, then the attribute must return null.

The commandHidden attribute must return true if the command's Hidden State is that the command is hidden, and false if the command is not hidden. If the element does not define a command, the attribute must return null.

The commandDisabled attribute must return true if the command's Disabled State is that the command is disabled, and false if the command is not disabled. This attribute is not affected by the command's Hidden State. If the element does not define a command, the attribute must return null.

The commandChecked attribute must return true if the command's Checked State is that the command is checked, and false if it is that the command is not checked. If the element does not define a command, the attribute must return null.

The ID facet is exposed by the id IDL attribute, the Hint facet is exposed by the title IDL attribute, and the AccessKey facet is exposed by the accessKeyLabel IDL attribute.

The commands attribute of the document's Document interface must return an HTMLCollection rooted at the Document node, whose filter matches only elements that define commands and have IDs.

4.11.6.2 Using the a element to define a command

An a element with an href attribute defines a command.

The Type of the command is "command".

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the string given by the element's textContent IDL attribute.

The Hint of the command is the value of the title attribute of the element. If the attribute is not present, the Hint is the empty string.

The Icon of the command is the absolute URL obtained from resolving the value of the src attribute of the first img element descendant of the element in tree order, relative to that element, if there is such an element and resolving its attribute is successful. Otherwise, there is no Icon for the command.

The AccessKey of the command is the element's assigned access key, if any.

The Hidden State of the command is true (hidden) if the element has a hidden attribute, and false otherwise.

The Disabled State facet of the command is true if the element or one of its ancestors is inert, and false otherwise.

The Checked State of the command is always false. (The command is never checked.)

The Action of the command, if the element has a defined activation behavior, is to run synthetic click activation steps on the element. Otherwise, it is just to fire a click event at the element.

4.11.6.3 Using the button element to define a command

A button element always defines a command.

The Type, ID, Label, Hint, Icon, Access Key, Hidden State, Checked State, and Action facets of the command are determined as for a elements (see the previous section).

The Disabled State of the command is true if the element or one of its ancestors is inert, or if the element's disabled state is set, and false otherwise.

4.11.6.4 Using the input element to define a command

An input element whose type attribute is in one of the Submit Button, Reset Button, Image Button, Button, Radio Button, or Checkbox states defines a command.

The Type of the command is "radio" if the type attribute is in the Radio Button state, "checkbox" if the type attribute is in the Checkbox state, and "command" otherwise.

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command depends on the Type of the command:

If the Type is "command", then it is the string given by the value attribute, if any, and a UA-dependent, locale-dependent value that the UA uses to label the button itself if the attribute is absent.

Otherwise, the Type is "radio" or "checkbox". If the element is a labeled control, the textContent of the first label element in tree order whose labeled control is the element in question is the Label (in DOM terms, this is the string given by element.labels[0].textContent). Otherwise, the value of the value attribute, if present, is the Label. Otherwise, the Label is the empty string.

The Hint of the command is the value of the title attribute of the input element. If the attribute is not present, the Hint is the empty string.

If the element's type attribute is in the Image Button state, and the element has a src attribute, and that attribute's value can be successfully resolved relative to the element, then the Icon of the command is the absolute URL obtained from resolving that attribute that way. Otherwise, there is no Icon for the command.

The AccessKey of the command is the element's assigned access key, if any.

The Hidden State of the command is true (hidden) if the element has a hidden attribute, and false otherwise.

The Disabled State of the command is true if the element or one of its ancestors is inert, or if the element's disabled state is set, and false otherwise.

The Checked State of the command is true if the command is of Type "radio" or "checkbox" and the element is checked attribute, and false otherwise.

The Action of the command, if the element has a defined activation behavior, is to run synthetic click activation steps on the element. Otherwise, it is just to fire a click event at the element.

4.11.6.5 Using the option element to define a command

An option element with an ancestor select element and either no value attribute or a value attribute that is not the empty string defines a command.

The Type of the command is "radio" if the option's nearest ancestor select element has no multiple attribute, and "checkbox" if it does.

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the value of the option element's label attribute, if there is one, or else the value of option element's textContent IDL attribute, with leading and trailing whitespace stripped, and with any sequences of two or more space characters replaced by a single U+0020 SPACE character.

The Hint of the command is the string given by the element's title attribute, if any, and the empty string if the attribute is absent.

There is no Icon for the command.

The AccessKey of the command is the element's assigned access key, if any.

The Hidden State of the command is true (hidden) if the element has a hidden attribute, and false otherwise.

The Disabled State of the command is true if the element is disabled, or if its nearest ancestor select element is disabled, or if it or one of its ancestors is inert, and false otherwise.

The Checked State of the command is true (checked) if the element's selectedness is true, and false otherwise.

The Action of the command depends on its Type. If the command is of Type "radio" then it must pick the option element. Otherwise, it must toggle the option element.

4.11.6.6 Using the menuitem element to define a command

A menuitem element that does not have a command attribute defines a command.

The Type of the command is "radio" if the menuitem's type attribute is "radio", "checkbox" if the attribute's value is "checkbox", and "command" otherwise.

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the value of the element's label attribute, if there is one, or the empty string if it doesn't.

The Hint of the command is the string given by the element's title attribute, if any, and the empty string if the attribute is absent.

The Icon for the command is the absolute URL obtained from resolving the value of the element's icon attribute, relative to the element, if it has such an attribute and resolving it is successful. Otherwise, there is no Icon for the command.

The AccessKey of the command is the element's assigned access key, if any.

The Hidden State of the command is true (hidden) if the element has a hidden attribute, and false otherwise.

The Disabled State of the command is true if the element or one of its ancestors is inert, or if the element has a disabled attribute, and false otherwise.

The Checked State of the command is true (checked) if the element has a checked attribute, and false otherwise.

The Action of the command, if the element has a defined activation behavior, is to run synthetic click activation steps on the element. Otherwise, it is just to fire a click event at the element.

4.11.6.7 Using the command attribute on menuitem elements to define a command indirectly

A menuitem element with a master command defines a command.

The Type of the command is the Type of the master command.

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the Label of the master command.

If the element has a title attribute, then the Hint of the command is the value of that title attribute. Otherwise, the Hint of the command is the Hint of the master command.

The Icon of the command is the Icon of the master command.

The AccessKey of the command is the element's assigned access key, if any.

The Hidden State of the command is the Hidden State of the master command.

The Disabled State of the command is the Disabled State of the master command.

The Checked State of the command is the Checked State of the master command.

The Action of the command is to invoke the Action of the master command.

4.11.6.8 Using the accesskey attribute on a label element to define a command

A label element that has an assigned access key and a labeled control and whose labeled control defines a command, itself defines a command.

The Type of the command is "command".

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the string given by the element's textContent IDL attribute.

The Hint of the command is the value of the title attribute of the element.

There is no Icon for the command.

The AccessKey of the command is the element's assigned access key.

The Hidden State, Disabled State, and Action facets of the command are the same as the respective facets of the element's labeled control.

The Checked State of the command is always false. (The command is never checked.)

4.11.6.9 Using the accesskey attribute on a legend element to define a command

A legend element that has an assigned access key and is a child of a fieldset element that has a descendant that is not a descendant of the legend element and is neither a label element nor a legend element but that defines a command, itself defines a command.

The Type of the command is "command".

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command is the string given by the element's textContent IDL attribute.

The Hint of the command is the value of the title attribute of the element.

There is no Icon for the command.

The AccessKey of the command is the element's assigned access key.

The Hidden State, Disabled State, and Action facets of the command are the same as the respective facets of the first element in tree order that is a descendant of the parent of the legend element that defines a command but is not a descendant of the legend element and is neither a label nor a legend element.

The Checked State of the command is always false. (The command is never checked.)

4.11.6.10 Using the accesskey attribute to define a command on other elements

An element that has an assigned access key defines a command.

If one of the earlier sections that define elements that define commands define that this element defines a command, then that section applies to this element, and this section does not. Otherwise, this section applies to that element.

The Type of the command is "command".

The ID of the command is the value of the id attribute of the element, if the attribute is present and not empty. Otherwise the command is an anonymous command.

The Label of the command depends on the element. If the element is a labeled control, the textContent of the first label element in tree order whose labeled control is the element in question is the Label (in DOM terms, this is the string given by element.labels[0].textContent). Otherwise, the Label is the textContent of the element itself.

The Hint of the command is the value of the title attribute of the element. If the attribute is not present, the Hint is the empty string.

There is no Icon for the command.

The AccessKey of the command is the element's assigned access key.

The Hidden State of the command is true (hidden) if the element has a hidden attribute, and false otherwise.

The Disabled State of the command is true if the element or one of its ancestors is inert, and false otherwise.

The Checked State of the command is always false. (The command is never checked.)

The Action of the command is to run the following steps:

1. If the element is focusable, run the focusing steps for the element.
2. If the element has a defined activation behavior, run synthetic click activation steps on the element.
3. Otherwise, if the element does not have a defined activation behavior, fire a click event at the element.

A dialog element without an open attribute specified should not be shown to the user. This requirement may be implemented indirectly through the style layer. For example, user agents that support the suggested default rendering implement this requirement using the CSS rules described in the rendering section.

When the show() method is invoked, the user agent must run the following steps:

1. If the element already has an open attribute, then abort these steps.

2. Add an open attribute to the dialog element, whose value is the empty string.

3. If the show() method was invoked with an argument, set up the position of the dialog element, using that argument as the anchor. Otherwise, set up the default static position of the dialog element.

---

Each Document has a stack of dialog elements known as the pending dialog stack. When a Document is created, this stack must be initialized to be empty.

When an element is added to the pending dialog stack, it must also be added to the top layer layer. When an element is removed from the pending dialog stack, it must be removed from the top layer. [FULLSCREEN]

When the showModal() method is invoked, the user agent must run the following steps:

1. Let subject be the dialog element on which the method was invoked.

2. If subject already has an open attribute, then throw an InvalidStateError exception and abort these steps.

3. If subject is not in a Document, then throw an InvalidStateError exception and abort these steps.

4. Add an open attribute to subject, whose value is the empty string.

5. If the showModal() method was invoked with an argument, set up the position of subject, using that argument as the anchor. Otherwise, set up the default static position of the dialog element.

6. Let subject's Document be blocked by the modal dialog subject.

7. Push subject onto subject's Document's pending dialog stack.

8. Let control be the first element in tree order that has an autofocus attribute specified and whose nearest ancestor dialog element is subject, if any.

   If there isn't one, then let control be the first element in tree order that is focusable and whose nearest ancestor dialog element is subject, if any.

   If there isn't one of those either but subject itself is focusable, then let control be subject. (This is unusual.)

9. If there is no control, then abort these steps.

10. Run the focusing steps for control.

If at any time a dialog element is removed from a Document, then if that dialog is in that Document's pending dialog stack, the following steps must be run:

1. Let subject be that dialog element and document be the Document from which it is being removed.

2. Remove subject from document's pending dialog stack.

3. If document's pending dialog stack is not empty, then let document be blocked by the modal dialog that is at the top of document's pending dialog stack. Otherwise, let document be no longer blocked by a modal dialog at all.

When the close() method is invoked, the user agent must close the dialog that the method was invoked on. If the method was invoked with an argument, that argument must be used as the return value; otherwise, there is no return value.

When a dialog element subject is to be closed, optionally with a return value result, the user agent must run the following steps:

1. If subject does not have an open attribute, then throw an InvalidStateError exception and abort these steps.

2. Remove subject's open attribute.

3. If the argument result was provided, then set the returnValue attribute to the value of result.

4. If subject is in its Document's pending dialog stack, then run these substeps:

   1. Remove subject from that pending dialog stack.

   2. If that pending dialog stack is not empty, then let subject's Document be blocked by the modal dialog that is at the top of the pending dialog stack. Otherwise, let document be no longer blocked by a modal dialog at all.

5. Queue a task to fire a simple event named close at subject.

The returnValue IDL attribute, on getting, must return the last value to which it was set. On setting, it must be set to the new value. When the element is created, it must be set to the empty string.

---

Canceling dialogs: When a Document's pending dialog stack is not empty, user agents may provide a user interface that, upon activation, queues a task to fire a simple event named cancel that is cancelable at the top dialog element on the Document's pending dialog stack. The default action of this event must be to check if that element has an open attribute, and if it does, close the dialog with no return value.

An example of such a UI mechanism would be the user pressing the "Escape" key.

---

The containing block of all dialog elements that are absolutely positioned must be the initial containing block.

All dialog elements are always in one of two modes: mundanely aligned, or magically aligned. When a dialog element is created, it must be placed in the mundanely aligned mode and the user agent must set up the default static position for that element, without an anchor.

When a user agent is to set up the default static position of an element subject without an anchor, if that element is being rendered, it must set up the element such that its top static position, for the purposes of calculating the used value of the 'top' property, is the value that would place the element's top margin edge as far from the top of the viewport as the element's bottom margin edge from the bottom of the viewport, if the element's height is less than the height of the viewport, and otherwise is the value that would place the element's top margin edge at the top of the viewport.

If there is a dialog element that is mundanely aligned and that is being rendered when its browsing context changes viewport width (as measured in CSS pixels), then the user agent must set up the default static position of all such elements in that browsing context again, still without anchors.

When a dialog element that is mundanely aligned starts being rendered, the user agent must set up the default static position of that element, without an anchor.

This top static position of a mundanely aligned dialog element must remain the element's top static position until the set up the default static position algorithm is once again invoked for that element. (The element's static position is only used in calculating the used value of the 'top' property in certain situations; it's not used, for instance, to position the element if its 'position' property is set to 'static'.)

When a user agent is to set up the position of an element subject using an anchor anchor, it must run the following steps:

1. If anchor is a MouseEvent object, then run these substeps:

   1. If anchor's target element does not have a rendered box, or is in a different document than subject, then let subject be mundanely aligned, set up the default static position of subject without an anchor, and abort the set up the position steps.

   2. Let anchor element be an anonymous element rendered as a box with zero height and width (so its margin and border boxes both just form a point), positioned so that its top and left are at the coordinate identified by the event, and whose properties all compute to their initial values.

   Otherwise, let anchor element be anchor.

2. Let subject be magically aligned to anchor element.

While an element A is magically aligned to an element B, the following requirements apply:

• If at any time either A or B cease having rendered boxes, A and B cease being in the same Document, or B ceases being earlier than A in tree order, let subject be mundanely aligned, and set up the default static position of subject without an anchor.

• A's 'position' property must compute to the keyword 'absolute-anchored' rather than whatever it would otherwise compute to (i.e. the 'position' property's specified value is ignored).

  The 'absolute-anchored' keyword's requirements are described below.

• The anchor points for A and B are defined as per the appropriate entry in the following list:

  If the computed value of 'anchor-point' is 'none' on both A and B

  The anchor points of A and B are the center points of their respective first boxes' border boxes.

  If the computed value of 'anchor-point' is 'none' on A and a specific point on B

  The anchor point of B is the point given by its 'anchor-point' property.

  If the anchor point of B is the center point of B's first box's border box, then A's anchor point is the center point of its first box's margin box.

  Otherwise, A's anchor point is on one of its margin edges. Consider four hypothetical half-infinite lines L1, L2, L3, and L4 that each start in the center of B's first box's border box, and that extend respectively through the top left corner, top right corner, bottom right corner, and bottom left corner of B's first box's border box. A's anchor point is determined by the location of B's anchor point relative to these four hypothetical lines, as follows:

  If the anchor point of B lies on L1 or L2, or inside the area bounded by L1 and L2 that also contains the points above B's first box's border box, then let A's anchor point be the horizontal center of A's bottom margin edge.

  Otherwise, if the anchor point of B lies on L3 or L4, or inside the area bounded by L4 and L4 that also contains the points below B's first box's border box, then let A's anchor point be the horizontal center of A's top margin edge.

  Otherwise, if the anchor point of B lies inside the area bounded by L4 and L1 that also contains the points to the left of B's first box's border box, then let A's anchor point be the vertical center of A's right margin edge.

  Otherwise, the anchor point of B lies inside the area bounded by L2 and L3 that also contains the points to the right of B's first box's border box; let A's anchor point be the vertical center of A's left margin edge.

  If the computed value of 'anchor-point' is a specific point on A and 'none' on B

  The anchor point of A is the point given by its 'anchor-point' property.

  If the anchor point of A is the center point of A's first box's margin box, then B's anchor point is the center point of its first box's border box.

  Otherwise, B's anchor point is on one of its border edges. Consider four hypothetical half-infinite lines L1, L2, L3, and L4 that each start in the center of A's first box's margin box, and that extend respectively through the top left corner, top right corner, bottom right corner, and bottom left corner of A's first box's margin box. B's anchor point is determined by the location of A's anchor point relative to these four hypothetical lines, as follows:

  If the anchor point of A lies on L1 or L2, or inside the area bounded by L1 and L2 that also contains the points above A's first box's margin box, then let B's anchor point be the horizontal center of B's bottom border edge.

  Otherwise, if the anchor point of A lies on L3 or L4, or inside the area bounded by L4 and L4 that also contains the points below A's first box's margin box, then let B's anchor point be the horizontal center of B's top border edge.

  Otherwise, if the anchor point of A lies inside the area bounded by L4 and L1 that also contains the points to the left of A's first box's margin box, then let B's anchor point be the vertical center of B's right border edge.

  Otherwise, the anchor point of A lies inside the area bounded by L2 and L3 that also contains the points to the right of A's first box's margin box; let B's anchor point be the vertical center of B's left border edge.

  If the computed value of 'anchor-point' is a specific point on both A and B

  The anchor points of A and B are the points given by their respective 'anchor-point' properties.

  The rules above generally use A's margin box, but B's border box. This is because while A always has a margin box, and using the margin box allows for the dialog to be positioned offset from the box it is annotating, B sometimes does not have a margin box (e.g. if it is a table-cell), or has a margin box whose position may be not entirely clear (e.g. in the face of margin collapsing and 'clear' handling of in-flow blocks).

  In cases where B does not have a border box but its border box is used by the algorithm above, user agents must use its first box's content area instead. (This is in particular an issue with boxes in tables that have 'border-collapse' set to 'collapse'.)

• When an element's 'position' property computes to 'absolute-anchored', the 'float' property does not apply and must compute to 'none', the 'display' property must compute to a value as described by the table in the section of CSS 2.1 describing the relationships between 'display', 'position', and 'float', and the element's box must be positioned using the rules for absolute positioning but with its static position set such that if the box is positioned in its static position, its anchor point is exactly aligned over the anchor point of the element to which it is magically aligned. Elements aligned in this way are absolutely positioned. For the purposes of determining the containing block of other elements, the 'absolute-anchored' keyword must be treated like the 'absolute' keyword.

The trivial example of an element that does not have a rendered box is one whose 'display' property computes to 'none'. However, there are many other cases; e.g. table columns do not have boxes (their properties merely affect other boxes).

If an element to which another element is anchored changes rendering, the anchored element will be be repositioned accordingly. (In other words, the requirements above are live, they are not just calculated once per anchored element.)

The 'absolute-anchored' keyword is not a keyword that can be specified in CSS; the 'position' property can only compute to this value if the dialog element is positioned via the APIs described above.

Elements positioned in this way are not clipped by the 'overflow' property of ancestors (nor moved by the resulting scrolling mechanisms), since the containing block is the initial containing block. Anchoring to an element that is so clipped (and shifted) can therefore result in unexpected effects (where the anchored element moves along with the clipped element, but isn't itself clipped).

User agents in visual interactive media should allow the user to pan the viewport to access all parts of a dialog element's border box, even if the element is larger than the viewport and the viewport would otherwise not have a scroll mechanism (e.g. because the viewport's 'overflow' property is set to 'hidden').

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The open IDL attribute must reflect the open content attribute.