Accessibility/WebAccessibilityAPI

Introduction

There's number of objectives on the web to improve accessibility and usability support. Web applications want to provide special support for their users, helping them navigate and perceive the content. The browser has a number of add-ons serving to improve accessibility support, for example, the app letting to navigate landmarks on the web page. These tasks require accessibility API similar to what desktop assistive technologies have.

Web accessibility API also allows for in-browser automated accessibility testing of web content, i.e. helpful for checking that HTML and other standards in the browser are accessible to all users.

On the other hand there's a growing need for making graphical content accessible. These are charts, graphs and other various visual forms that are drawn using HTML canvas or SVG. There's also a tendency to use HTML canvas element in place of DOM because of performance matter, here's an example. All markup in the example is defined in JavaScript and there is a need for a non-DOM accessibility solution to make the content accessible.

Web accessibility API

The API provides bunch of interfaces that are used to express the web content the way the assistive technologies (AT) knows how to deal with. In other words each piece of semantically meaningful content has associated accessible element the AT operates at. The interfaces allow to receive accessible element properties, traverse the hierarchy and interact with content.

Also the API provides a way to extend existing semantics of the markup and add new semantics for inaccessible content like canvas drawings.

Accessible element

This is a basic interface providing an access to the accessible element.

interface AccessibleElement {
  attribute DOMString role;

  attribute DOMString name;
  attribute DOMString description;
  attribute DOMString value;

  readonly attribute StateSet states;

  readonly attribute AttributeMap attributes;

  readonly attribute sequence<DOMString> patterns;
  readonly attribute Object toPattern(DOMString type);

  readonly attribute RelationMap relations;
  AccessibleElement? relativeOf(DOMString type);

  readonly attribute ActionMap actions;
  void activate(DOMString action);
  readonly attribute InteractionMap intreactions;

  readonly attribute AccesibleElement? parent;
  readonly attribute AccesibleElement? firstChild;
  readonly attribute AccesibleElement? lastChild;
  readonly attribute AccesibleElement? nextSibling;
  readonly attribute AccesibleElement? previousSibling;
  readonly attribute AccessibleChildren children;

  readony attribute Node? DOMNode;
  attribute AccessibleSource? source;
};

Node interface extension

Accessible element can be requested from a DOM node if the DOM node is accessible, i.e. it expresses meaningful semantics to the assistive technology.

partial interface Node {
  AccessibleElement? accessibleElement;

  A11eElement? a11ement; // nicer and shorter verison? // another version: accElement, accEl
};

Role

AccessibleElement .role

Returns accessible role as a string. Examples: "button", "menu", "textfield".

Text properties

AccessibleElement .name

Returns accessible name.

AccessibleElement .description

Returns accessible description.

AccessibleElement .value

Returns accessible value if applicable.

States

AccessibleElement .states

Returns a StateSet live object for all accessible states of an accessible element.

Accessible states of an accessible element are presented by the StateSet object. It is a live object, i.e. if the states of the accessible element change then the object reflects that. Implementation is not required to compute any state until requested.

interface StateSet {
  readonly setlike<DOMString>;

  boolean hasAnyOf(DOMString ... states);
  boolean hasAllOf(DOMString ... states);
};

StateSet .hasAnyOf

Returns true if any of the given states are present on the accessible element.

StateSet .hasAllOf

Returns true if all given states are present on the accessible element.

Example of a script that logs to console all accessible element states.

var accEl = document.getElementById("foo").accessibleElement;
for (let state of accElm.states) {
  console.log(state);
}

Attributes

The accessible element may support a number of attributes to express the semantics that cannot be described by base properties of an accessible element.

AccessibleElement .attributes

Returns a AttributeMap live object for all object attributes exposed on the accessible element. If accessible element attributes are changed then the object reflects the actual state of attributes. The implementation is not required to pre-compute any of the attributes.

AttributeMap

AttributeMap is an interface aimed to work with {name, value} pairs of accessible attributes.

interface AttributeMap {
  readonly maplike<DOMString, any>;

  boolean hasAnyOf(Object map); // Needs more work
  boolean hasAllOf(Object map); // Needs more work
};

AttributeSet .hasAnyOf

Return true if any of the object properties matches to the object attributes.

AttributeSet .hasAllOf

Return true if all of the object properties matches to the object attributes.

Each property of the object is a string or an array of strings. If array of strings is given then the object attribute is expected to have any of given values, empty array means the attribute value doesn't matter for match. String value and null values are treated as single element array or empty array correspondingly.

Example #1.

var attrs = {
  live: [ "assertive", "polite" ],
  relevant: "show",
  busy: null
};

var matched = accEl.hasAllOf(attrs);
// Matches if the ccessible element has "live" object attribute of
// "assertive" or "polite" values, has "relevant" object attribute of
// "show" value, and it has "busy" attribute.

Example #2. Process autocomplete.

var accEl = document.getElementById("foo").a11ement;

var checkObj = { autocomplete: [ "list", "both" ] };
if (accEl.attributes.hasAllOf(checkObj)) {
  doAutocomplete();
}

// Alternatively you can do
var autocomplete = accEl.attributes.get("autocomplete");
if (["list", "both"].indexOf(autocomplete) != -1) {
  doAutocomplete();
}

Attribute list

Set of exposed attributes depends on semantics of the element. As an example, typical attributes are:

DOMString autocomplete;

Exposed on text fields. Values are list, none, inline, both.

DOMString live;

Points that the accessible is live region. Values are assertive and polite.

DOMString relevant;

Lists all notifications that qualifies for live region.

Patterns

A pattern is a collection of attributes or methods that expresses an accessible element semantics and compliments what the AccessibleElement interface provides. The patterns concept is quite similar to attributes but tends to be more powerful. Basically, it is an alternative implementation of COM's queryInterface.

AccessibleElement .patterns

Returns a list of all patterns supported by the accessible element.

Object .toPattern(DOMString type)

Returns an object for the pattern of given type if supported by an accessible element.

See patterns section for details.

Relations

AccessibleElement? .relativeOf(DOMString type)

Returns related accessible element of the given relation type. If relation points to multiple accessible elements, the first one is returned.

RelationMap .relations

Returns a map of relation types as keys and related accessible elements as values.

interface RelationMap {
  maplike<DOMString, sequence<AccessibleElement>>;
};

Relation types

This is a typical list of relation types that may be exposed by an accessible element. Note, the accessible element may support custom relations as well.

labelfor

Referred accessible element is an element that is labelled by this accessible element.

labelledby

Referred accessible element is a label for this accessible element.

descriptionfor

Referred accessible element is an element that is described by this accessible element.

describedby

Referred accessible element is a description for this accessible element.

widget

Return a widget the item belongs to.

parent

Return a logical parent of the item. This relation may be useful for ARIA structures if the browser doesn't convert DOM flat structure into accessible hierarchical tree.

Examples

Console logging of all headers of the grid cell and name of the grid itself.

  var headers = cell.relations.get("labelledby");
  for (let header of headers) {
    console.log(header.role, header.name);
  }

  var grid = cell.relativeOf("widget");
  console.log(grid.name);

Actions

An accessible element may support actions that can be invoked on it. For example, jump on a link or press on a button, or it can be generic purpose actions like scroll or focus. Certain accessible actions may take optional parameters.

AccessibleElement .actions

Returns a ActionSet object of actions exposed by the accessible element. The returned object is not live, i.e. it is not updated if the accessible element actions change.

AccessibleElement .activate(in DOMString name, in optional any param)

Invokes the accessible action.

Parameters:

name

action name to invoke

param

used to provide extra context for the action

Performs the given accessible action on the accessible element.

Action list

activate

Exposed on accessible elements that may be activated. Accessible elements may use other names for this action to emphasize the semantics they expose. For example, jump on links, press on buttons, check and uncheck on checkboxes. Accessible element may provide more than one action. For example, tree item can provide select/unselect as its primary action and expand/collapse as secondary. Tree column may implement sort action.

focus

Focus on an accessible element. May be different from "activate" action, for example, in case of buttons where "activate" means press.

scroll

Scrolls an accessible element into view, optionally takes coordinates relative the element to scroll the view to

Parameter:

delta of ScrollDelta

drag and drop

Used to start dragging and dropping the content related to the accessible element the action is invoked on.

zoomin / zoomout

Zooms in/out the content corresponding the accessible the action is invoked on.

Parameter:

ratio of double, optional

undo / redo

Performs clipboard operations on the accessible element of the editable content.

ScrollDelta param

directory ScrollDelta {
  int x;
  int y;
  int z;
  DOMString mode;
};

ScrollDelta structure describes a scroll change within the accessible element, the scroll change can be expressed in different units, for example, it can be in pixels or in pages. The accessible element may provide its custom set of supported modes which has to be described in taxonomy.

ScrollDelta .x, .y, .z

3d coordinates in the units provided by mode, y and z might be optional depending on the unit.

ScrollDelta .mode

A unit the coordinates are measured in. Can be on of the following values:

pixel - x, y and z (optional) specifies how many pixles to scroll (2d or 3d)

line - x specifies how many lines to scroll

paragraph - x specifies how many paragraphs to scroll

page - x specifies how many pages to scroll

ActionSet

Accessible actions are presented by ActionSet map like object of pairs { action name, action object }.

interface ActionSet {
  readonly maplike<DOMString, Action>;
};

Each action is presented by Action interface.

interface Action {
  stingifier readonly attribute name;
  readonly attribute description;
  readonly InteractionSet interactions(optional DOMString device);
};

Action .name

Action name

Action .description

Localized action description

Action .interactions

Set of interactions (like keyboard shortcuts or mouse gestures) to invoke the action.

Parameters

device (optional)

device name like 'keyboard' or 'touchscreen' the interactions should be returned for.

Here's an example of script announcing available actions on the accessible element.

var set = accElm.actions;
if (set.size() == 0) {
  say("no actions on accessible element");
} else {
  say("accessible element has " + set.size() + " actions");
  set.forEach(action => say(action.description));
}

Interactions

A widget may support actions that can be invoked through variety of devices or by AT like screen readers. The way the action is triggered by the user is described by the interaction. Examples of user interactions are swipe, a mouse gesture to toggle a switch control; space, a key to toggle a checkbox. Access keys (alt+letter) and keyboard shortcuts (ctrl+shift+letter) used to activate the widgets are also examples of interactions. Complex widgets like grid control may support several actions, for example ctrl+A for selectall action, ArrowDown/ArrowUp keys for movedown/moveup actions, ArrowRight/ArrowLeft keys for movenext/moveprev actions.

The AT may need to know the used shortcuts to avoid possible conflicts or to supply a native way to invoke the action. For example, if presentation supports swipe gesture to navigate the slides then the AT can announce this info for the user.

AccessibleElement .interactions

Return an InteractionSet object describing all interactions that can be performed with the accessible element.

interface InteractionSet {
  readonly maplike<DOMString, Interaction>;

  boolean hasAnyOf(sequence<DOMString> interactions);
  readonly attribute InteractionSet allOf(DOMString action, optional DOMString device);
};

AccessibleElement .hasAnyOf

Return true if any of given interaction are in use by the widget.

Parameters:

interaction

list of interactions

AccessibleElement .allOf

Return a list of all interactions with the accessible element as InteractionSet used to invoke the given action on the given device.

Parameters:

action

action name

device

a device name, examples of supported values are "keyboard", "mouse", "touchpad".

interface Interaction {
  stingifier readonly attribute description;
  readonly attribute device;
};

Here's an example of the screen reader announcing all interactions of swipe action.

var actions = accElm.actions;
actions.forEach(action => { var set = action.interactions;
                            say(action.name);
                            set.values().forEach(item => say(item.description));
                          });

Parent-child relations

The interface provides a bunch of methods to provide access to parent/child relations between accessible elements.

AccessibleElement .parent

Returns the parent accessible element if any.

AccessibleElement .firstChild

Returns the first child accessible element if any.

AccessibleElement .lastChild

Returns the last child accessible element if any.

AccessibleElement .nextSibling

Returns the next sibling accessible element if any.

AccessibleElement .previousSibling

Returns the previous sibling accessible element if any.

AccessibleElement .children

Return AccessibleChildren collection of all child accessible elements.

interface AccessibleChildren {
  iterable<AccessibleElement>;
};

Questions/concerns:

• Consider to move "widget" relation to "widget" IDL attribute (property) under this section.

Accessible providers

The accessible element can be created with different kinds of sources. Most of the accessible elements are created for a DOM element to expose its semantics to the assistive technology. In some cases the browser may not have underlying DOM node for the content. In this case the accessible object is created, without having a corresponding DOM element. In some cases a web author may need to change the semantics for existing elements, add new semantics or create a new accessible subtree to make content accessible. In this case the author needs to describe desired semantics in JavaScript. See Making the content accessible in JavaScript for details.

AccessibleElement .DOMNode

Returns a DOM node associated with the accessible element if any. Accessible element does not have a DOM node when it is either not based on it, or when it has been detached from the DOM node. The first case happens for browser-specific implementations or for trees created in JavaScript. The second case happens when the JavaScript holds reference to the object longer than the object life cycle.

AccessibleElement .source

Returns AccessibleSource if present, refers to an accessible source, used to override accessible properties of the element.

Content traversal

This section describes how to traverse accessible content, for example, if you need to navigate a document by headings or navigate a paragraph by words.

A key concept of the content traversal is accessible position which describes a "location" in the document content. A position can be placed on an accessible element, for example, on a button. It can be placed before or after an accessible element, or inside of the accessible element text, for example, at the end of the first line of a paragraph.

The position can be moved forward or backward inside the document to the content complying with the given criteria. For example, criteria can be described verbally like "move the position forward to next heading" or "move the position one word back".

Accessible Position

[Constructor(AccessibleElement anchor, optional Offset offset, optional AccessibleElement root),
 Constructor(DOMNode container, long offset, optional AccessibleElement root),
 Constructor(DOMPoint point, optional AccessibleElement root),
 Constructor(AccessiblePos pos, optional AccessibleElement root)]
interface AccessiblePos {
  AccessiblePos? move(AccessibleElement anchor, Offset offset);
  AccessiblePos? move(DOMNode container, long offset);
  AccessiblePos? move(DOMPoint);
  AccessiblePos? move(Where where, Controller);

  AccessiblePos? search(Where where, Controller);

  readonly attribute AccessibleElement root;
  readonly attribute AccessibleElement? anchor;
  readonly attribute Offset offset;
  DOMRangeBound toDOM();
};

typedef AccessiblePos A11ePos; // Short and nice name? // Maybe accPos

Construction

AccessiblePos .Constructor(AccessibleElement, Offset, AccessibleElement)

Constructs the accessible position.

Parameters

elm of AccessibleElement

anchor accessible element of the position

offset of Offset, optional

an offset relative to the anchor element. If not provided then at or afterbegin is used depending on element type.

root of AccessibleElement, optional

a root of subtree containing the position. If not provided then parent document is used.

typedef long or OffsetLiterals Offset;
enum OffsetLiterals {
  "before",
  "afterbegin",
  "at",
  "beforeend",
  "after"
};

OffsetLiterals .before

Used to set the accessible position right before the accessible element beginning

OffsetLiterals .afterbegin

Used to set the accessible position right after the accessible element beginning

OffsetLiterals .at

Used to set the accessible position at the accessible element

OffsetLiterals .beforeend

Used to set the accessible position right before the accessible element ending

OffsetLiterals .after

Used to set the accessible position right after the accessible element ending

AccessiblePos .Constructor(DOMPoint, AccessibleElement)

Constructs the accessible position.

Parameters

point of DOMPoint

a point, the accessible position should be set at

root of AccessibleElement, optional

a root of subtree containing the position. If not provided then parent document is used.

AccessiblePos .Constructor(DOMNode, long, AccessibleElement)

Constructs the accessible position from (DOMNode, offset) pair.

Parameters

node of DOMNode

the container node

offset of long

offset within the container node

root of AccessibleElement, optional

a root of subtree containing the position. If not provided then parent document is used.

AccessiblePos .Constructor

Constructs the accessible position equal to given position.

Parameters

pos of AccessiblePos

accessible position to copy

Examples:

var pos1 = new A11ePos(new DOMPoint(x, y), document.accessibleElement);

var pos2 = new A11ePos(button.accessibleElement, "at");
var pos3 = new A11ePos(paragraph.accessibleElement, "afterbegin");

var pos4 = new A11ePos(pos2);

Move through the content

AccessiblePos .move(AccessilbeElement, Offset)

Move the accessible position to the given anchor and offset.

Parameters

element of AccessibleElement

the anchor

offset of Offsest, optional

offset relative the anchor

Return itself.

AccessiblePos .move(DOMNode, long)

Move the accessible position to the given DOM node at given offset.

Parameters

node of DOMNode

the anchor

offset of long

offset relative the anchor

Return itself.

AccessiblePos .move(DOMPoint)

Move the accessible position in the content. Returns true if succeeded.

Parameters

point of DOMPoint

the point the accessible position should be moved to.

Return itself.

AccessiblePos .move(Where, Controller)

Move the accessible position to the content. Returns true if succeeded.

Parameters

where of Where

where the search should be performed

controller of Controller

function describing a match

Return itself.

AccessiblePos .search(Where, Controller)

Finds the accessible position relative the current one. Returns new instance.

Parameters

where of Where

where the search should be performed

controller of Controller

function describing a match

Return new instance.

enum Where { "forward",
             "backward",
             "cyclic forward",
             "cyclic backward",
             "tofirst",
             "tolast",
             "left",
             "right",
             "up",
             "down",
             "above",
             "under",
             "closest"
};

Where .forward

Search after the accessible position.

Where .backward

Search backwards from the accessible position.

Where .cyclic forward

Search forwards from the accessible position in cycle.

Where .cyclic backward

Search backwards from the accessible position in cycle.

Where .tofirst

Search for a first match inside the root.

Where .tolast

Search backwards (from last to first element) for a first match inside the root.

Where .left

Search for a match left to the position.

Where .right

Search for a match right to the position.

Where .up

Search for a match up from the position.

Where .down

Search for a match down from the position.

Where .above

Search for a match above the position.

Where .under

Search for a match under the position.

Where .closest

Search for a geometrically closest match to the position.

Examples:

function controller(elm) { }
var pos1 = new A11ePos(new DOMPoint(x, y), a11edoc).move("forward", controller);

var pos2 = new A11ePos(elm, "at").move("backward", controller);

var pos3 = new A11ePos(pos).move("forward", controller);
or
var pos2 = pos2.search("forward", controller);

Matching function

callback Controller = SearchTerm sequence<SearchTerm> (AccessibleElement);

Controller

A matching function used to define an algorithm of how the position is moved through the document.

Returns

a search term(s)

enum SearchTerm {
  "char",
  "word",
  "sentence",
  "line",
  "paragraph",
  "change",
  "bit",

  "at",
  "skipsubtree",
  "stop",
  "next"
};

SearchTerm .char

Used to move the position one char long.

SearchTerm .word

Used to move the position to the next word.

SearchTerm .sentence

Used to move the position to the next sentence.

SearchTerm .line

Used to move the position to beginning of next line or end of previous line.

SearchTerm .paragraph

Used to move the position to beginning of next/previous paragraph.

SearchTerm .change

Used to move the position to next change either of text attribute or new accessible.

SearchTerm .bit

Used to move the position to next/previous navigable position. For example, from accessible start inside of it to right before accessible outside of it or moving from end of this line to start of next line in case of soft line break lines.

SearchTerm .at

Used to set the position at the element. Search is stopped.

SearchTerm .stop

Search is discontinued after other search terms are processed.

SearchTerm .skipsubtree

Subtree of traversed element is ignored for next step.

SearchTerm .next

Used to continue the search if other search terms weren't succeed. Default option.

Example. Traverse a paragraph by words.

var p = document.getElementById("p").a11ement;
var pos1 = new A11ePos(p, "afterbegin"), pos2 = null;
while (pos2 = pos1.search("forward", () = > { return "word"; })) {
  console.log(pos2.textInBetween(pos1));
  pos1 = pos2;
}

<p>Mozilla is a <a href="">free-software</a> community which
produces the <a href="">Firefox web browser</a>.</p>
<p>Mo<a href="">zilla</a>.</p>

the log is "Mozilla ", "is ", "a ", "free-", "software ", "community", "which ", "produces ", "the ", "Firefox ", "web ", "browser."

The log is "It ", "is ", "Mozilla".

Examples.

var pos1 = new A11ePos(document.getElementById("a").a11ement);
var pos2 = pos1.search("forward", () = > { return [ "word", "stop" ];});
var pos3 = pos1.search("forward", () => { return "word"; });

<p>Click <a id="a">here</a>, fellow</p>

pos2 and pos3 points after "here" word in the link.

<p>Click <a id="a"><img src=""></a>, fellow</p>

pos2 equals to pos1, pos3 points after "fellow" word in the link.

Example. Navigate by widgets and structures, and by words in case of text.

function controller(aEl)
{
  var role = document.taxonOf("role", aEl.role);
  if (role.is("widget")
    return [ "beforeend" ];

  if (role.is("structure")
    return "at";

  // Reenters with next accessible if it cannot move to the next word within this accessible.
  if (role.is("textcontainer"))
    return "word";

  return "next";
}

pos.move("next", finder);

Other methods

AccessiblePos .root

Returns the accessible element the position movement is restricted to.

AccessiblePos .anchor

Returns the accessible element the position is at or contained by.

AccessiblePos .offset

Return an offset of the accessible position, either numeric or text.

AccessiblePos .toDOM

Returns DOMRangeBound object which is a pair of DOM node and content offset relative it.

dictionary DOMRangeBound {
  DOMNode node;
  long offset;
};

Virtual cursor

Virtual cursor is accessible position attached to the accessible document. It's unique per document.

partial interface AccessibleDocument {
  AccesiblePos cursor;
};

Whenever the virtual cursor position is changed then cursor_moved event is fired.

Questions/concerns

• Do we need *is* method right on AccessibleElement or should we have Role interface having that method or should AccessibleElement return role taxa as a role.
• Do we need to have "inContextOf" on AccessibleElement to check what the accessilbe belongs to. Note, native implementation doing some cache may be faster than tree traversal. If we need it then it's worth to consider *is* method too.
• Do we need isAnyOf() method additionally?
• Do we need to compare() method to compare two positions
• Ensure that the virtual cursor is survivable. If it ends up in the hidden sub-tree for example. If subtee gets destroyed then cursor should be moved. If AccessiblePos doesn't have matching function then it's unclear where it should be moved.
• Also take into account walking to/from parent/child frames
• Should it be outlined as moved?
• Yura: we should have async API for search.

[ Promise variant if we had async API].

function getText() {
  var p = new A11ePos().move("first", a => a.role == "paragraph");
  var startPos;
  return p.then(function(pos) { 
    startPos = pos; 
    return pos.search("forward", a => "line"); }).
  then(function(endPos) {
    return startPos.textInBetween(endPos);
  };
}
getText().then(function(text) { /* Say text */ })

Text

partial interface AccessiblePos {
  DOMString textInBetween(AccessiblePos pos);
  readonly attribute AttributeSet textAttributes;
};

AccessibleDocument .textInBetween

Returns the text enclosed between this and given accessible positions.

Example how to get first line of the first encountered paragraph:

var startPos = new A11ePos().move("first", a => a.role == "paragraph");
var endPos = startPos && startPos.search("forward", a => [ "line", "stop" ]);
var text = startPos.textInBeetween(startPos, endPos);

say(text);

Text attributes

AccessibleDocument .textAttributes

Returns a AttributeSet of all text attributes presented at this accessible position.

List of text attributes [to complete]

color

Text color

background-color

Background color of text

Example

<p id="p">hello <b>bold</b></p>

var pos = new A11ePos(document.getElementById("p").a11ement, 7);
p.textAttributes.get("font-weight") == 700; // true

Caret and selection

Document accessible provides bunch of methods to operate on text selection and caret.

partial interface AccessibleDocument {
  attribute AccessiblePos caret;
  attribute AccessiblePos? selectionStart;
  attribute AccessiblePos? selectionEnd;
};

AccessibleDocument .caret

Returns accessible position for the caret.

AccessibleDocument .selectionStart

Get/set selection start.

AccessibleDocument .selectionEnd

Get/set selection end.

Example

var pos = new A11ePos(new DOMPoint(event.x, event.y));
document.a11ement.selectionStart = pos;
var pos2 = new A11ePos(pos.anchor.nextSibling);
document.a11ement.selectionEnd = pos;

Example #2. Connection with DOM selection.

var sel = window.getSelection();
var pos1 = new A11ePos(sel.anchorNode, sel.anchorOffset);
var pos2 = new A11ePos(sel.focusNode, sel.focusOffset);

var p1 = pos1.toDOM();
sel.getRangeAt(0).setStart(p1.node, p1.offset);
var p2 = pos2.toDOM();
sel.getRangeAt(0).setEnd(p2.node, p2.offset);

Geometry

The section contains bunch of methods and approaches to deal with web page geometry.

Outline

AT can outline a position or a range enclosed between two positions. This feature is useful to track stuff like virtual cursor.

partial interface AccessibleDocument {
  void outline(AccessiblePos pos1, optional AccessiblePos pos2);
  void clearOutlines();
};

AccessibleDocument .outline

Outlines a position if second position is omitted. If second position is omitted then outlines a collapsed range.

AccessibleDocument .clearOutlines

Removes all drawn outlines.

Geometrical navigation

AT can scan the web page by moving the position geometrically up/down/left/right.

var scanner
{
  start: function() {
    if (this.stopped)
      return;

    var nextpos = this.pos.move("right", this.controller);
    if (!nextpos)
      nextpos = this.pos.move("down", this.controller);

    if (nextpos) {
      document.outline(nextpos);
      window.setTimeout(this.start.bing(this), 1000);
    }
  },
  stop: function() {
    this.stopped = true;
  },

  controller: function(aEl) {
    var role = document.a11ement.taxonOf("role", aElm.role);
    if (role.is("widget"))
      return "at";

    return "next";
  }
  pos: new A11ePos(new DOMPoint(0, 0)),
  stopped: false
}

Hit testing

The author can do accessible position to match the accessible element of given criteria at given position.

<script>
function onmove(aEvent)
{
  // Announce name of the accessible element pointed by mouse.
  var atcursor = new A11ePos(new DOMPoint(event.x, event.y));
  say(atcursor.anchor.name);

  // Locate a nearest control.
  var atwidget = atcursor.search("closest", a => document.taxonOf("role", a.role).is("widget"));
  if (atwidget)
    say(atwidget.name);
};
</script>

<body onmove="onmove(event);></body>

Questions/concerns

• Outline: it's tricky to style that but maybe document or accessible should decide how it should be styled, for example, black outline on black background.
• Do we need a way to return coordinates of accessible position's edge points?
• Do we need a method to calculate distance between two positions?
• Do we need a method to calculate boundaries (aka containing rect)?
• Do we need a method to check whether the given point is contained by a range?

Events

The accessible element may fire various number of events. You can add event listener on document accessible to handle all events from accessible elements belonging to the document.

callback AccessibleEventListener (AccessibleEvent event);

partial interface AccessibleDocument {
  void on(DOMString eventType, AccessibleEventListener listener);
  void off(DOMString eventType, AccessibleEventListener listener);
};

AccessibleDocument .on

Adds event listener of the given event type.

AccessibleDocument .off

Removes event listener of the given event type.

All event objects implement AccessibleEvent interface.

interface AccessibleEvent {
  readonly attribute DOMString type;
  readonly attribute AccessibleElement target;
  readonly attribute AttributeMap attributes;
};

AccessibleEvent .type

Event type.

AccessibleEvent .target

Event type accessible element target.

AccessibleEvent .attributes

Extra attributes used to describe the event. Used for certain event types.

Example. Live regions processing.

function handleLiveRegion(aEvent)
{
  var attrs =
    { live: "assertive", relevant: "change:name" };

  if (aEvent.attributes.hasAllOf(attrs)) {
    say(aEvent.target.name);
  }
}
document.a11ement.on("change:name");

Event types

[section needs to be completed]

"focus"

Fired when accessible element is focused.

"caretmove"

Fired when caret is moved within the document.

"change:name"

Fired when name property is changed.

Patterns

Controls may expose own patterns to expose their semantics if it cannot be expressed by accessible element properties.

Value controls

Control elements having numeric value like slider, progressbar, meter or numberbox implement AccessibleValue inteface.

interface AccessibleValue {
  readonly attribute long max;
  readonly attribute long min;
  readonly attribute long low;
  readonly attribute long high;
  readonly attribute long optimum;
  readonly attribute long step;
  readonly attribute long value;
};

Group info

Item elements provides information about their group and their position in the group.

interface AccessibleGroupInfo {
  readonly attribute long count;
  readonly attribute long index;
  readonly attribute long level;
};

Tables

A cell of tables and grids provide the interface for 2 dimensional navigation. Cell headers can be obtained by labelledby relation.

interface AccessibleTableCell {
  readonly attribute unsigned long rowIndex;
  readonly attribute unsigned long colIndex;

  readonly attribute unsigned long rowSpan;
  readonly attribute unsigned long colSpan;
};

AccessibleTableCell .rowIndex

Return row index of the cell.

AccessibleTableCell .colIndex

Return column index of the cell.

AccessibleTableCell .rowSpan

Return number of rows that the cell is spanned to.

AccessibleTableCell .colSpan

Return number of columns that the cell is spanned to.

Multiprocessing

Accessible tree is scoped to a process, i.e. you can have seamless accessible tree between all document frames until it violates security policy. In case of multiprcess frames you have to use standard mechanism to communicate between processes.

When the virtual cursor reaches a frame start or end and it was attempted to move it further then message is sent to a process where the virtual cursor would go in case of single process.

Questions/concerns

• what messaging mechanism should be used
• should it be allowed to move from iframe to parent document?

Make the content accessible

You can tweak the accessible tree right in JS. You can do both:

• extend the native semantics of the DOM element by overriding its accessible properties,
• create whole accessible trees for inaccessible content, for example, for graphics, charts in a canvas.

Direct change of accessible properties

Accessible element properties can be altered right on the accessible element. Changing accessible property will result in accessible events if needed. For example:

<button id="button"></button>

<script>
var div = document.getElementById("button").a11ement;
div.name = "click me";
div.description = "test accessibility";
</script>

Accessible name and description are changed for the accessible button, "name changed" and "description changed" events are fired. Note, it changes neither visual appearance of the button nor DOM button element properties. If affects on accessible element of the button only.

States

parital interface StateSet {
  void set(DOMString ... states);
  void unset(DOMString ... states);
  void toggle(DOMString state);
};

StateSet .set

Set given states on the accessible element.

StateSet .unset

Unset given states from the accessible element.

StateSet .toggle

Toggle the state on the accessible element.

Example.

<div role="checkbox" onclick="disableEnable();">
<div role="listbox" id="listbox"/>

<script>
function disableEnable()
{
  var listbox = document.getElementById("listbox").a11ement;
  listbox.states.toggle("disabled");
}
</script>

Attributes

You can add/change/delete attributes on the accessible element. For example,

var listbox = document.getElementById("listbox").a11ement;
listbox.attributes.set("autocomplete", "list");

Relations

partial interface RelationMap {
  void add(DOMString relation, AccessibleElement element);
  void remove(DOMString relation, AccessibleElement element);
};

RelationMap .add

Puts the accessible element into relations.

RelationMap .remove

Breaks the given relations between these accessible elements.

var label = document.getElementById("label");
var control = document.getElementById("control").a11ement;
control.relations.add("labelledby", label.a11yment);

[to do actions/interactions and patterns]

Extend the semantic via accessible source

Alternatively you can extend native semantic of a DOM element by connecting it to an accessible source, the object describing accessible element. If the DOM node is not accessible then setting the accessible source on it makes it accessible. Otherwise the given source lands on top of the existing accessible object.

partial interface Node {
  attribute AccessibleSource? accessibleSource;
};

If the DOM node is accessible then you can provide accessible source by setting it up right in AccessibleElement.

partial interface AccessibleElement {
  attribute AccessibleSource? source;
};

The source object used to describe the accessible element implements AccessibleSource interface which basically portrays properties of AccessibleElement interface.

callback interface AccessibleSource {
  readonly attribute DOMString role;
  readonly attribute sequence<DOMString> states;

  readonly attribute DOMString name;
  readonly attribute DOMString description;
  readonly attribute DOMString value;

  readonly attribute object attributes;
  readonly attribute object relations;

  readonly attribute sequence<DOMString> patterns;
  object toPattern(DOMString);

  readonly attribute DOMString text;
  object textAttributesAt(Offset);

  readonly attribute object actions;
  void activate(DOMString action);

  AccessibleElement? element;
};

Basics

AccessibleSource .role

AccessibleSource .states

AccessibleSource .name

AccessibleSource .description

AccessibleSource .value

These properties corresponds to properties of AccessibleElement interface.

Example #1. Override accessible name of the button.

If the author needs to change accessible name of the HTML button

<button id="btn">press me</button>

then he can do this either with ARIA

document.getElementById("btn").setAttribute("aria-label", "new name");

or set it directly on the accessible element

document.getElementById("btn").a11ement.name = "new name";

or by setting its accessible source

document.getElementById("btn").accessibleSource = { name: "new name"; }

These are two equivalent approaches that can be used same time and mixed with each other the way that better fits the author's intent. It's important to keep in mind though that the accessible source has a priority over ARIA attributes. Setting up the accessible source doesn't operate on DOM and thus it's supposed to be more performant as extra layer is excluded between the author and accessibility engine. Another benefit of setting the accessible source is improved code readability since the author is able to point multiple properties at once in declarative way. For example,

<script>
document.getElementById("btn").accessibleSource = {
  name: "new name",
  description: "new description",
  states: [ "disabled" ]
};
</script>

Attributes and relations

AccessibleSource .attributes

A map of attribute names and their values applied to the accessible element

AccessibleSource .relations

A map of relation types and relations for this accessible element. The bunch of types may be used to point out related elements. You can use either Node, AccessibleElement or AccessibleSource objects. In latter case the object has to be in the accessible tree, in other words, it has to have associated accessible element when relation is poked by AT.

Example #2. Adding semantics.

The accessible source can be also used when the author wants to add semantics to semantically neutral element. For example, the author can turn an ordinal HTML div element into listbox control:

<label id="label">Names:</label>
<div id="list"></div>

<script>
var listboxSource = {
  role: "listbox",
  relations: {
    "labelledby": [ document.getElementById("label") ]
  }
};
var listboxNode = document.getElementById("list");
listboxNode.accessibleSource = listboxSource;
</script>

It's worth to notice that accessible source approach like ARIA doesn't require the author to provide comprehensive description, i.e. correlating properties may be skipped. For example, presented labelledby relation makes unnecessary to provide accessible name. The browser should handle this on its own.

Example #3. Lazy computations.

The previous example may look bulkier than its ARIA equivalent:

var listboxNode = document.getElementById("list");
listboxNode.setAttribute("role", "listbox");
listboxNode.setAttribute("aria-labelledby", "label");

but its benefit is it allows accessible properties lazy computation. An accessible property whose value isn't changed during whole life cycle of its accessible element or which doesn't require an accessible event on its change is good candidate for lazy computation. For example the previous example may be modified this way:

var listboxSource = {
  role: "listbox",
  relations: {
    get labelledby() {
      return document.querySelectorAll("*[listboxlabel]");
    }
  }
};

In this case the labelledby relation getter is more powerful than its ARIA version and all computations are running iff somebody inquired the relation.

Patterns

AccessibleSource .patterns

Returns a list of all patterns implemented by the accessible element.

AccessibleSource .toPattern

Returns an object for the given pattern. If the method is not provided then the source object has to implemented itself all claimed patterns.

Example #4. Implement patterns.

var slider = {
  role: "slider",
  patterns: [ "value" ],
  toPattern: function(aName) {
    return aName == "value" ? this : null;
  },

  min: 0,
  max: 100,
  value: 0
};

Text

AccessibleSource .text

Return the text.

AccessibleSource .textAttributesAt

Returns object of { name: value } pairs representing text attributes at given offset within the accessible element text.

Example #5. Text content.

While text should be normally in the content, there are cases when the author has to provide it for non-text content. For example, this technique can be used to turn HTML image into text.

<div id="container"></div>

<script>
var elm = document.getElementById("container");
elm.innerHTML = "I " + <img src="love.png"> + "you";

var psource = {
  role: "paragraph",
  text: "I love you",
  textAttributesAt: function(aOffset) {
    if (aOffset >= 5 && aOffset <= 7) {
      return { color: "red" }; // "love" is red
    }
  }
};
elm.accessibleSource = psource;
</script>

Actions

AccessibleSource .actions

Return all supported actions on the accessible element.

AccessibleSource .activate

Invoke the given action.

Parameters

action

the action name to invoke

Example #6. Describe actions and interactions.

function listitem(aName)
{
  return {
    role: "treeitem",
    name: aName,
    actions: {
      select: {
        interactions: [ { mouse: "click" } ]
      }
    }
  };
};

In case if interactions cannot be provided then the accessible source have to implement activate method to invoke actions.

Inheritance

AccessibleSource .element

Refers to the accessible element implementation as there were no accessible source attached to it. Set by the browser when the accessible source is attached to the accessible element.

Example. Lazy extension of accessible relations.

var source = {
  get relations() {
    var r = this.element.relations;
    r.add("labelledby", document.querySelector(this.selector));
    return r;
  },
  element: null,
  selector: ""
};
document.getElementById("button").accessibleSource = source;

Change the accessible tree

The web developer can alter existing accessible tree including subtree creations and deletions. This can be done either by AccessibleSource object assigned to the accessible element or by direct manipulations on the accessible tree.

Direct tree alteration

Accessible source object can be used to create new accessible element and insert it into the tree. If DOM node is used as a source then associated subtrees of accessible elements within the DOM node are reparanted, i.e. they are removed from children of its current accessible parent and added as children of new parent. Accessible element can be removed from children.

partial interface AccessibleChildren {
  void append(AccessibleSource or DOMNode ... children);
  void insertBefore(AccessibleElement before, AccessibleSource or DOMNode ... children);
  void insertAfter(AccessibleElement after, AccessibleSource or DOMNode ... children);
  void replace(AccessibleSource or DOMNode ... children);
  void remove(AccessibleElement ... children);
  void clear();
};

Children .append

Appends accessible elements provided by sources as children to the accessible element.

Children .insertBefore

Inserts accessible elements provided by sources before the given child.

Children .insertAfter

Inserts accessible elements provided by sources after the given child.

Children .replace

Replaces existing children accessible elements within elements provided by sources.

Children .remove

Removes accessible elements from the accessible tree.

Children .clear

Removes all child accessible elements.

Example

<input id="input" role="combobox">
<div id="list" role="listbox">
  <div role="listitem">choice1</div>
  <div role="listitem">choice1</div>
</div>

<script>
var input = document.getElementById("input").a11ement;
input.children.replace(document.getElementById("list"));
</script>

Build up the tree via accessible source

You can provide child accessible elements for the given accessible element by assigning AccessibleSource object to it. This approach allows to create whole portions of the accessible tree for the content having no underlying DOM nodes.

partial interface AccessibleSource {
  sequence<AccessibleSource or DOMNode> children;
};

AccessibleSource .children

Return collection of accessible children. If AccessibleSource is provided then it's used to create a new accessible element and insert it as a child. If accessible DOMNode is provided then its accessible element is used, if inaccessible DOMNode is provided then its accessible children if any are used.

Example. Add list with items into the tree.

var tree = {
  role: "listbox",
  name: "colors",
  children: [
    {
      role: "listitem",
      states: "focusable",
      name: "red"
    },
    {
      role: "listitem",
      states: "focusable",
      name: "green"
    }
  ]
};
divNode.accessibleSource = tree;

Canvas hit regions

Canvas hit region options are extended by a11ement which is used to connect accessible element with hit region. Whenever hit region is activated then activate method of connected AccessibleSource object is called with no argument provided.

Say you have a canvas with drawn button. In order to make it accessible you can create accessible button right in JS.

var source = {
  role: "button",
  name: "test",
  actions: {
    click: {
      interactions: {
        mouse: "click"
      }
    }
  },
  onactivated: function(aEvent) {
    if (aEvent.region == "button")
      this.element.set("focused");
    else
      this.element.states.unset("focused");
  }
  element: null // set by the browser
};

canvasNode.addHitRegion({ id: "button", a11ement: source });
canvasNode.addEventListener("click", source.onactivated.bind(source));

Feedback notifications

Accessible element may be notified of any kind of event, including about property change when managed by connected source object. When notified the browser may fire accessible events and update its cache if necessary. Decision whether to notify the browser or not should be based on the accessible events model. In other words if a property change in native markup causes accessible event then it's quite likely the event is expected for similar accessible source change.

partial interface AccessibleElement {
  void notifyOf(DOMString eventType, optional Object attrs);
};

AccessibleElement .notifyOf

Called when source has been changed to notify the host accessible element about property change. The browser is responsible to fire proper accessible events and update its internal representation.

Parameters

eventType of DOMString

The event type to fire.

attrs

Attributes describing the event. See event types for attributes variety.

If the above listbox gets disabled then you have to reflect that in its state:

var listboxSource = {
  role: "listbox",
  name: "breed list",
  states: "disabled",

  get element() { return this.elm; }
  set element(aElm) {
    this.elm = aElm;
    this.elm.notifyOf("change:name");
    this.elm.notifyOf("change:states", { added: "disabled" });
  }
};

function onfocus(aEvent) {
  // Change accessible properties when element is focused.
  // The change requires accessible events.
  aEvent.target.accessibleSource = listboxSource;
}

Implied semantics

The author has to be less verbose as possible. If a role usually implies number of sets or attributes then they are supposed to set so the authur doesn't need to list them. For example if listitem is selectable then you don't have to point selectable state on it, in other words, these code snippets are equivalent:

var source = {
  role: "listitem"
};

var source = {
  role: "listitem",
  states: [ "selectable" ]
};

The implied semantics is described by semantics provides.

Conflicts

[to be done conflicts between native semantics, ARIA and this API]

Sniffing

In order to make optimization the content provider has to know whether accessibility consumer is active. The provider can add a callback for deploy and conceal event types which will be triggered when consumer appears/gets inactive.

function deployA11y() {
  // build accessible tree or update existing one
  // keep the tree updated
}
function stopA11y() {
  // stop keeping the accessible tree updated
}
document.a11ement.on("deploy", deployA11y);
document.a11ement.on("conceal", stopA11y);

Semantics providers

The author of the content which is not a standard HTML has to provide content semantics to make the content accessible. In other words, he has to describe a semantic vocabulary that will be used to give the content specific meaning. For example, if you are ebook publisher then you will need a bunch of new roles like 'bibliography' or 'appendix'. If you do operate with complex widgets then likely you will need to describe its states, attributes or relations.

partial interface AccessibleDocument {
  void import(DOMString name, Object taxonomy);
};

AccessibleDocument .import

Import the taxonomy.

Parameters

name of DOMString

taxonomy name like role or states

taxonomy of Object

a collection of taxa and their properties.

Throws

if there's names conflict or there are invalid relations between taxonomies then an exception should be thrown [todo].

Depending on taxonomy type, a taxa may be described by specific fields and be connected with other taxonomies. For example, DPUB semantics may be described this way:

document.accessibleElement.import("role",
{
  appendix: {
    parents: "section",
    landmark: "appendix",
    description: "appendix"
  },
  bibliography: {
    parents: "list",
    landmark: "bibliography",
    owns: "biblioentry",
    description: "bibliography"
  },
  biblioentry: {
    parents: "listitem",
    description: "reference"
  }
}

This approach is alternative to DPUB ARIA extensions. Any web app operating with DPUB content can import JS file describing the taxonomies and the browser can expose the content properly to the assistive technology. No special support on the browser side is required.

In short reasons to provide taxonomy are:

• The browser automatically maps the content to desktop accessibility APIs;
• The web accessibility technology knows how to expose the content;
• Adds implied accessibility support for the content;
• Automatic content validation.

Taxonomies

You can get hierarchical relations between roles, states, actions or anything else that has a taxonomy. For example, if the web author introduces a role 'checklistitem' which is compound from two roles 'checkbox' and 'listitem', i.e it inherits properties of both of them then the web author should integrate the new role into existing role hierarchy. Taxonomies are scoped by AccessibleDocument which provides bunch of methods to get them.

partial interface AccessibleDocument {
  Taxon? taxonOf(in DOMString taxonomy, in DOMString name);
  Taxonomy? taxonomyOf(DOMString name);
};

AccessibleDocument .taxonOf

Returns a taxon for the given taxon name of requested taxonomy.

Parameters

taxonomy

a taxonomy name

name

name of requested taxa like 'button' in case of 'role' taxonomy

AccessibleDocument .taxonomyOf

Return an object describing requested taxonomy.

Parameters

name

a taxonomy name

interface Taxon {
  stringifier readonly attribute DOMString name;
  readonly attribute DOMString description;

  readonly attribute sequence<Taxon> parentTaxa;
  readonly attribute sequence<Taxon> childTaxa;

  bool is(DOMString or Taxon base);

  readonly attribute AttributeMap attributes;
  readonly attribute RelationMap relations;
};

Taxon .name

Return name of the taxa

Taxon .description

Return localized description of the taxa

Taxon .parentTaxa

Return list of taxa the taxon is inherited from.

Taxon .childTaxa

Return list of taxa that is inherited from this taxon.

Taxon .is

Return true if given taxon name is in base of the taxon.

Taxon .attributes

Return a map between attribute names and attribute values specific for the taxon.

Taxon .relations

Return a map between relation types and related taxa specific for the taxon.

Example #1. Handle new role as it was known.

var taxa = {
  checklistitem: {
    parents: [ "checkbox", "menuitem" ]
  }
};
document.a11ement.import("role", taxa);

var taxon = document.a11ement.taxonOf("role", "checklistitem");
if (taxon.is("menuitem")) {
  // process menuitem selection
}

Example #2. Checking whether role is a navigational landmark.

var taxon = document.a11ement.taxonOf("role", "main");
var isLandmark = taxon.attributes.has("landmark");

Example #3. Getting states specific to the role.

var taxon = document.a11ement.taxonOf("role", "listbox");
print(listboxRole.relations.get("states"))); // prints "focusable"

Managing the taxonomies

interface Taxonomy {
  Taxon? taxonOf(DOMString name);
  readonly attribute sequence<Taxon> rootTaxa;

  void addTaxon(DOMString name, sequence<Taxon or DOMString> baseTaxa);
  void removeTaxon(Taxon taxon);
};

Taxonomy .getTaxon

Return a taxon object for the given name.

Parameters

name

taxon name

Taxonomy .rootTaxa

Return list of base taxa

Taxonomy .addTaxon

Adds new taxon into the taxonomy.

Parameters

name

a new taxon name

baseTaxa

a list of taxa the taxon is inherited from

Taxonomy .removeTaxon

Remove the given taxon and all taxa inherited from it from the taxonomy.

Parameters

taxon

taxon object

Taxonomy types

Each taxonomy is described means of its own interface which is used to define the hierarchy and interconnections between taxonomies. If taxon refers to taxa of other taxonomy type then a string describing the reference may be in form of "taxon:modifier", where taxon is a name of related taxon and modifier is an extra information about how the taxa are interconnected. For example, role taxon may be connected to "live" taxon of attributes taxonomy by placing "live:assertive" value in attributes field which means that the role supports "live" object attribute and its default value is "assertive" on it.

Roles

dictionary RoleTaxon {
  DOMString landmark;
  DOMString description;

  sequence<DOMString> parents;
  sequence<DOMString> owns;
  sequence<DOMString> states;
  Object attributes;
};

RoleTaxon .landmark

Navigation landmark name if applicable.

RoleTaxon .description

Localized role description.

RoleTaxon .parents

List of roles the role is inherited from.

RoleTaxon .owns

List of roles allowed in children of the role. Used for validation.

RoleTaxon .states

List of states allowed on the role. Optional modifier is "default" which points out that the state is exposed on the role until explicitly specified otherwise. See implied semantics.

RoleTaxon .attributes

List of attributes supported by the role. Default value of the attribute may be specified as modifier. For example, "live:polite" points out that "live" object attribute has "polite" value by default. If default value is not specified then it's taken from referred attribute taxon description.

Example. ARIA textbox role

var taxa = {
  widget: { }
  input: {
    parents: [ "widget" ],
    states: [ "focusable:default", "focused" ]
  },
  textbox: {
    description: "text field",
    parents: [ "input" ],
    states: [
      "singleline:default", "multiline",
      "editable", "readonly",
      "required"
    ],
    attributes: [ "autocomplete" ]
  }
};

Example. ARIA log role.

var taxa = {
  region: {
  },
  log: {
    parents: [ "region" ],
    attributes: [ "live:polite" ]
  }
};

States

dictionary StateTaxon {
  DOMString description;
  sequence<DOMString> dependents;
  DOMString opposite;
}

StateTaxon .description

Localized taxa description.

StateTaxon .dependents

List of all dependent states. For example, "focused" state always accompanied by "focusable" state.

StateTaxon ..opposite

An opposite state if any. Opposite state are mutually exclusive. For example, if "vertical" state is applied then "horizontal" is not and vice versa.

Example.

var taxa = {
  focusable: {
    dependents: [ "focused" ]
  },
  focused: { },
  singleline: {
    opposite: "multiline"
  },
  multiline: {
    opposite: "singleline"
  },
  readonly: { },
  editable: { },
  required: { }
};

Attributes

interface AttributeTaxon {
  DOMString description;
  sequence<DOMString> values;
  DOMString default;
};

AttributeTaxon .description

Localized description of the taxon

AttributeTaxon .values

List of all possible values of the attrbiute

AttributeTaxon .default

Default attribute value. Takes a place if role taxa pointing to it doesn't have own default value of it.

Example

var taxa = {
  autocomplete: {
    values: [ "none", "list", "inline", "both" ],
    default: "none"
  },
  live: {
    values: [ "none", "polite", "assertive" ],
    default: "none"
  }
};

HTML and beyond

This doc introduces common patterns to express the semantics of markup languages to accessibility. Markup specifics is not a target for this doc in general. Each markup specification has to take care to describe their accessibility stuff in terms of this API.

Extensibility

The web application might need to extend default taxonomies to express the new semantics. For example, the web service publishing music sheets can introduce new characteristics like role, states, etc to describe music sheet content. However the web application should take care to explain new characteristic by extending default taxonomies, i.e. by describing the connection between old and new characteristics. That will resolve any backward compatibility issue, so if the consumer doesn't know about new roles then he can still figure out a closest match it's aware about. For example, if the web app author introduces "x-redbutton' and provides a role taxonomy for it saying this is an extension of 'button' role, then the consumer unfamiliar with 'x-redbutton' role will treat it as a button.

The author should follow name convention to avoid potential collisions with future additions into the spec predefined lists. Thus all names should be prefixed by 'x-' like 'x-redbutton' from example above.

Music sheet example

To make a music sheet accessible the web app may introduce bunch of new roles, attributes and relations:

roles:
  'sheet' - inherited from 'section'
  'note' - inherited from 'image', describes the note

role 'sheet' attributes:
  instrument: DOMString,
  tempo: number/DOMString
  clef: DOMString

role 'note' attributes:
  key: enum { C, D, E, F, G, A, H },
  alteration: enum { none, flat, sharp },
  octave: enum { ... },
  duration: number,
  effects: sequence<DOMString>, // tremolo, bend

role 'note' relations:
  crescendo: [note, ...] a list a notes the crescendo is applied to
  diminuendo: [note, ...] a list a notes the diminuendo is applied to