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dmazzoni	2f48975	2017-02-16 03:39:16	[diff] [blame]	1	# Accessibility Overview
				2
				3	Accessibility means ensuring that all users, including users with disabilities,
				4	have equal access to software. One piece of this involves basic design
				5	principles such as using appropriate font sizes and color contrast,
				6	avoiding using color to convey important information, and providing keyboard
				7	alternatives for anything that is normally accomplished with a pointing device.
				8	However, when you see the word "accessibility" in a directory name in Chromium,
				9	that code's purpose is to provide full access to Chromium's UI via external
				10	accessibility APIs that are utilized by assistive technology.
				11
				12	Assistive technology here refers to software or hardware which
				13	makes use of these APIs to create an alternative interface for the user to
				14	accommodate some specific needs, for example:
				15
				16	Assistive technology includes:
				17
				18	* Screen readers for blind users that describe the screen using
				19	synthesized speech or braille
				20	* Voice control applications that let you speak to the computer,
				21	* Switch access that lets you control the computer with a small number
				22	of physical switches,
				23	* Magnifiers that magnify a portion of the screen, and often highlight the
				24	cursor and caret for easier viewing, and
				25	* Assistive learning and literacy software that helps users who have a hard
				26	time reading print, by highlighting and/or speaking selected text
				27
				28	In addition, because accessibility APIs provide a convenient and universal
				29	way to explore and control applications, they're often used for automated
				30	testing scripts, and UI automation software like password managers.
				31
				32	Web browsers play an important role in this ecosystem because they need
				33	to not only provide access to their own UI, but also provide access to
				34	all of the content of the web.
				35
				36	Each operating system has its own native accessibility API. While the
				37	core APIs tend to be well-documented, it's unfortunately common for
				38	screen readers in particular to depend on additional undocumented or
				39	vendor-specific APIs in order to fully function, especially with web
				40	browsers, because the standard APIs are insufficient to handle the
				41	complexity of the web.
				42
				43	Chromium needs to support all of these operating system and
				44	vendor-specific accessibility APIs in order to be usable with the full
				45	ecosystem of assistive technology on all platforms. Just like Chromium
				46	sometimes mimics the quirks and bugs of older browsers, Chromium often
				47	needs to mimic the quirks and bugs of other browsers' implementation
				48	of accessibility APIs, too.
				49
				50	## Concepts
				51
				52	While each operating system and vendor accessibility API is different,
				53	there are some concepts all of them share.
				54
				55	1. The tree, which models the entire interface as a tree of objects, exposed
				56	to assistive technology via accessibility APIs;
				57	2. Events, which let assistive technology know that a part of the tree has
				58	changed somehow;
				59	3. Actions, which come from assistive technology and ask the interface to
				60	change.
				61
				62	Consider the following small HTML file:
				63
				64	```
				65	<html>
				66	<head>
				67	<title>How old are you?</title>
				68	</head>
				69	<body>
				70	<label for="age">Age</label>
				71	<input id="age" type="number" name="age" value="42">
				72	<div>
				73	<button>Back</button>
				74	<button>Next</button>
				75	</div>
				76	</body>
				77	</html>
				78	```
				79
				80	### The Accessibility Tree and Accessibility Attributes
				81
				82	Internally, Chromium represents the accessibility tree for that web page
				83	using a data structure something like this:
				84
				85	```
				86	id=1 role=WebArea name="How old are you?"
				87	id=2 role=Label name="Age"
				88	id=3 role=TextField labelledByIds=[2] value="42"
				89	id=4 role=Group
				90	id=5 role=Button name="Back"
				91	id=6 role=Button name="Next"
				92	```
				93
				94	Note that the tree structure closely resembles the structure of the
				95	HTML elements, but slightly simplified. Each node in the accessibility
				96	tree has an ID and a role. Many have a name. The text field has a value,
				97	and instead of a name it has labelledByIds, which indicates that its
				98	accessible name comes from another node in the tree, the label node
				99	with id=2.
				100
				101	On a particular platform, each node in the accessibility tree is implemented
				102	by an object that conforms to a particular protocol.
				103
				104	On Windows, the root node implements the IAccessible protocol and
				105	if you call IAccessible::get_accRole, it returns ROLE_SYSTEM_DOCUMENT,
				106	and if you call IAccessible::get_accName, it returns "How old are you?".
				107	Other methods let you walk the tree.
				108
				109	On macOS, the root node implements the NSAccessibility protocol and
				110	if you call [NSAccessibility accessibilityRole], it returns @"AXWebArea",
				111	and if you call [NSAccessibility accessibilityLabel], it returns
				112	"How old are you?".
				113
				114	The Linux accessibility API, ATK, is more similar to the Windows APIs;
				115	they were developed together. (Chrome's support for desktop Linux
				116	accessibility is unfinished.)
				117
				118	The Android accessibility API is of course based on Java. The main
				119	data structure is AccessibilityNodeInfo. It doesn't have a role, but
				120	if you call AccessibilityNodeInfo.getClassName() on the root node
				121	it returns "android.webkit.WebView", and if you call
				122	AccessibilityNodeInfo.getContentDescription() it returns "How old are you?".
				123
				124	On Chrome OS, we use our own accessibility API that closely maps to
				125	Chrome's internal accessibility API.
				126
				127	So while the details of the interface vary, the underlying concepts are
				128	similar. Both IAccessible and NSAccessibility have a concept of a role,
				129	but IAccessible uses a role of "document" for a web page, while NSAccessibility
				130	uses a role of "web area". Both IAccessible and NSAccessibility have a
				131	concept of the primary accessible text for a node, but IAccessible calls
				132	it the "name" while NSAccessibility calls it the "label", and Android
				133	calls it a "content description".
				134
				135	Historical note: The internal names of roles and attributes in
				136	Chrome often tend to most closely match the macOS accessibility API
				137	because Chromium was originally based on WebKit, where most of the
				138	accessibility code was written by Apple. Over time we're slowly
				139	migrating internal names to match what those roles and attributes are
				140	called in web accessibility standards, like ARIA.
				141
				142	### Accessibility Events
				143
				144	In Chromium's internal terminology, an Accessibility Event always represents
				145	communication from the app to the assistive technology, indicating that the
				146	accessibility tree changed in some way.
				147
				148	As an example, if the user were to press the Tab key and the text
				149	field from the example above became focused, Chromium would fire a
				150	"focus" accessibility event that assistive technology could listen
				151	to. A screen reader might then announce the name and current value of
				152	the text field. A magnifier might zoom the screen to its bounding
				153	box. If the user types some text into the text field, Chromium would
				154	fire a "value changed" accessibility event.
				155
				156	As with nodes in the accessibility tree, each platform has a slightly different
				157	API for accessibility events. On Windows we'd fire EVENT_OBJECT_FOCUS for
				158	a focus change, and on Mac we'd fire @"AXFocusedUIElementChanged".
				159	Those are pretty similar. Sometimes they're quite different - to support
				160	live regions (notifications that certain key parts of a web page have changed),
				161	on Mac we simply fire @"AXLiveRegionChanged", but on Windows we need to
				162	fire IA2_EVENT_TEXT_INSERTED and IA2_EVENT_TEXT_REMOVED events individually
				163	on each affected node within the changed region, with additional attributes
				164	like "container-live:polite" to indicate that the affected node was part of
				165	a live region. This discussion is not meant to explain all of the technical
				166	details but just to illustrate that the concepts are similar,
				167	but the details of notifying software on each platform about changes can
				168	vary quite a bit.
				169
				170	### Accessibility Actions
				171
				172	Each native object that implements a platform's native accessibility API
				173	supports a number of actions, which are requests from the assistive
				174	technology to control or change the UI. This is the opposite of events,
				175	which are messages from Chromium to the assistive technology.
				176
				177	For example, if the user had a voice control application running, such as
				178	Voice Access on Android, the user could just speak the name of one of the
				179	buttons on the page, like "Next". Upon recognizing that text and finding
				180	that it matches one of the UI elements on the page, the voice control
				181	app executes the action to click the button id=6 in Chromium's accessibility
				182	tree. Internally we call that action "do default" rather than click, since
				183	it represents the default action for any type of control.
				184
				185	Other examples of actions include setting focus, changing the value of
				186	a control, and scrolling the page.
				187
				188	### Parameterized attributes
				189
				190	In addition to accessibility attributes, events, and actions, native
				191	accessibility APIs often have so-called "parameterized attributes".
				192	The most common example of this is for text - for example there may be
				193	a function to retrieve the bounding box for a range of text, or a
				194	function to retrieve the text properties (font family, font size,
				195	weight, etc.) at a specific character position.
				196
				197	Parameterized attributes are particularly tricky to implement because
				198	of Chromium's multi-process architecture. More on this in the next section.
				199
				200	## Chromium's multi-process architecture
				201
				202	Native accessibility APIs tend to have a functional interface, where
				203	Chromium implements an interface for a canonical accessible object that
				204	includes methods to return various attributes, walk the tree, or perform
				205	an action like click(), focus(), or setValue(...).
				206
				207	In contrast, the web has a largely declarative interface. The shape
				208	of the accessibility tree is determined by the DOM tree (occasionally
				209	influenced by CSS), and the accessible semantics of a DOM element can
				210	be modified by adding ARIA attributes.
				211
				212	One important complication is that all of these native accessibility APIs
				213	are synchronous, while Chromium is multi-process, with the contents of
				214	each web page living in a different process than the process that
				215	implements Chromium's UI and the native accessibility APIs. Furthermore,
				216	the renderer processes are sandboxed, so they can't implement
				217	operating system APIs directly.
				218
				219	If you're unfamiliar with Chrome's multi-process architecture, see
				220	[this blog post introducing the concept](
				221	https://blog.chromium.org/2008/09/multi-process-architecture.html) or
				222	[the design doc on chromium.org](
				223	https://www.chromium.org/developers/design-documents/multi-process-architecture)
				224	for an intro.
				225
				226	Chromium's multi-process architecture means that we can't implement
				227	accessibility APIs the same way that a single-process browser can -
				228	namely, by calling directly into the DOM to compute the result of each
				229	API call. For example, on some operating systems there might be an API
				230	to get the bounding box for a particular range of characters on the
				231	page. In other browsers, this might be implemented by creating a DOM
				232	selection object and asking for its bounding box.
				233
				234	That implementation would be impossible in Chromium because it'd require
				235	blocking the main thread while waiting for a response from the renderer
				236	process that implements that web page's DOM. (Not only is blocking the
				237	main thread strictly disallowed, but the latency of doing this for every
				238	API call makes it prohibitively slow anyway.) Instead, Chromium takes an
				239	approach where a representation of the entire accessibility tree is
				240	cached in the main process. Great care needs to be taken to ensure that
				241	this representation is as concise as possible.
				242
				243	In Chromium, we build a data structure representing all of the
				244	information for a web page's accessibility tree, send the data
				245	structure from the renderer process to the main browser process, cache
				246	it in the main browser process, and implement native accessibility
				247	APIs using solely the information in that cache.
				248
				249	As the accessibility tree changes, tree updates and accessibility events
				250	get sent from the renderer process to the browser process. The browser
				251	cache is updated atomically in the main thread, so whenever an external
				252	client (like assistive technology) calls an accessibility API function,
				253	we're always returning something from a complete and consistent snapshot
				254	of the accessibility tree. From time to time, the cache may lag what's
				255	in the renderer process by a fraction of a second.
				256
				257	Here are some of the specific challenges faced by this approach and
				258	how we've addressed them.
				259
				260	### Sparse data
				261
				262	There are a lot of possible accessibility attributes for any given
				263	node in an accessibility tree. For example, there are more than 150
				264	unique accessibility API methods that Chrome implements on the Windows
				265	platform alone. We need to implement all of those APIs, many of which
				266	request rather rare or obscure attributes, but storing all possible
				267	attribute values in a single struct would be quite wasteful.
				268
				269	To avoid each accessible node object containing hundreds of fields the
				270	data for each accessibility node is stored in a relatively compact
				271	data structure, ui::AXNodeData. Every AXNodeData has an integer ID, a
				272	role enum, and a couple of other mandatory fields, but everything else
				273	is stored in attribute arrays, one for each major data type.
				274
				275	```
				276	struct AXNodeData {
				277	int32_t id;
				278	AXRole role;
				279	...
				280	std::vector<std::pair<AXStringAttribute, std::string>> string_attributes;
				281	std::vector<std::pair<AXIntAttribute, int32_t>> int_attributes;
				282	...
				283	}
				284	```
				285
				286	So if a text field has a placeholder attribute, we can store
				287	that by adding an entry to `string_attributes` with an attribute
				288	of ui::AX_ATTR_PLACEHOLDER and the placeholder string as the value.
				289
				290	### Incremental tree updates
				291
				292	Web pages change frequently. It'd be terribly inefficient to send a
				293	new copy of the accessibility tree every time any part of it changes.
				294	However, the accessibility tree can change shape in complicated ways -
				295	for example, whole subtrees can be reparented dynamically.
				296
				297	Rather than writing code to deal with every possible way the
				298	accessibility tree could be modified, Chromium has a general-purpose
				299	tree serializer class that's designed to send small incremental
				300	updates of a tree from one process to another. The tree serializer has
				301	just a few requirements:
				302
				303	* Every node in the tree must have a unique integer ID.
				304	* The tree must be acyclic.
				305	* The tree serializer must be notified when a node's data changes.
				306	* The tree serializer must be notified when the list of child IDs of a
				307	node changes.
				308
				309	The tree serializer doesn't know anything about accessibility attributes.
				310	It keeps track of the previous state of the tree, and every time the tree
				311	structure changes (based on notifications of a node changing or a node's
				312	children changing), it walks the tree and builds up an incremental tree
				313	update that serializes as few nodes as possible.
				314
				315	In the other process, the Unserialization code applies the incremental
				316	tree update atomically.
				317
				318	### Text bounding boxes
				319
				320	One challenge faced by Chromium is that accessibility clients want to be
				321	able to query the bounding box of an arbitrary range of text - not necessarily
				322	just the current cursor position or selection. As discussed above, it's
				323	not possible to block Chromium's main browser process while waiting for this
				324	information from Blink, so instead we cache enough information to satisfy these
				325	queries in the accessibility tree.
				326
				327	To compactly store the bounding box of every character on the page, we
				328	split the text into inline text boxes, sometimes called text runs.
				329	For example, in a typical paragraph, each line of text would be its own
				330	inline text box. In general, an inline text box or text run contians a
				331	sequence of text characters that are all oriented in the same direction,
				332	in a line, with the same font, size, and style.
				333
				334	Each inline text box stores its own bounding box, and then the relative
				335	x-coordinate of each character in its text (assuming left-to-right).
				336	From that it's possible to compute the bounding box
				337	of any individual character.
				338
				339	The inline text boxes are part of Chromium's internal accessibility tree.
				340	They're used purely internally and aren't ever exposed directly via any
				341	native accessibility APIs.
				342
				343	For example, suppose that a document contains a text field with the text
				344	"Hello world", but the field is narrow, so "Hello" is on the first line and
				345	"World" is on the second line. Internally Chromium's accessibility tree
				346	might look like this:
				347
				348	```
				349	staticText location=(8, 8) size=(38, 36) name='Hello world'
				350	inlineTextBox location=(0, 0) size=(36, 18) name='Hello ' characterOffsets=12,19,23,28,36
				351	inlineTextBox location=(0, 18) size=(38, 18) name='world' characterOffsets=12,20,25,29,37
				352	```
				353
				354	### Scrolling, transformations, and animation
				355
				356	Native accessibility APIs typically want the bounding box of every element in the
				357	tree, either in window coordinates or global screen coordinates. If we
				358	stored the global screen coordinates for every node, we'd be constantly
				359	re-serializing the whole tree every time the user scrolls or drags the
				360	window.
				361
				362	Instead, we store the bounding box of each node in the accessibility tree
				363	relative to its offset container, which can be any ancestor. If no offset
				364	container is specified, it's assumed to be the root of the tree.
				365
				366	In addition, any offset container can contain scroll offsets, which can be
				367	used to scroll the bounding boxes of anything in that subtree.
				368
				369	Finally, any offset container can also include an arbitrary 4x4 transformation
				370	matrix, which can be used to represent arbitrary 3-D rotations, translations, and
				371	scaling, and more. The transformation matrix applies to the whole subtree.
				372
				373	Storing coordinates this way means that any time an object scrolls, moves, or
				374	animates its position and scale, only the root of the scrolling or animation
				375	needs to post updates to the accessibility tree. Everything in the subtree
				376	remains valid relative to that offset container.
				377
				378	Computing the global screen coordinates for an object in the accessibility
				379	tree just means walking up its ancestor chain and applying offsets and
				380	occasionally multiplying by a 4x4 matrix.
				381
				382	### Site isolation / out-of-process iframes
				383
				384	At one point in time, all of the content of a single Tab or other web view
				385	was contained in the same Blink process, and it was possible to serialize
				386	the accessibility tree for a whole frame tree in a single pass.
				387
				388	Today the situation is a bit more complicated, as Chromium supports
				389	out-of-process iframes. (It also supports "browser plugins" such as
				390	the `<webview>` tag in Chrome packaged apps, which embeds a whole
				391	browser inside a browser, but for the purposes of accessibility this
				392	is handled the same as frames.)
				393
				394	Rather than a mix of in-process and out-of-process frames that are handled
				395	differently, Chromium builds a separate independent accessibility tree
				396	for each frame. Each frame gets its own tree ID, and it keeps track of
				397	the tree ID of its parent frame (if any) and any child frames.
				398
				399	In Chrome's main browser process, the accessibility trees for each frame
				400	are cached separately, and when an accessibility client (assistive
				401	technology) walks the accessibility tree, Chromium dynamically composes
				402	all of the frames into a single virtual accessibility tree on the fly,
				403	using those aforementioned tree IDs.
				404
				405	The node IDs for accessibility trees only need to be unique within a
				406	single frame. Where necessary, separate unique IDs are used within
				407	Chrome's main browser process. In Chromium accessibility, a "node ID"
				408	always means that ID that's only unique within a frame, and a "unique ID"
				409	means an ID that's globally unique.
				410
				411	## Blink
				412
				413	Blink constructs an accessibility tree (a hierarchy of [WebAXObject]s) from the
sashab	b498325a	2017-05-17 01:18:46	[diff] [blame]	414	page it is rendering. WebAXObject is the public API wrapper around [AXObject],
				415	which is the core class of Blink's accessibility tree. AXObject is an abstract
dmazzoni	2f48975	2017-02-16 03:39:16	[diff] [blame]	416	class; the most commonly used concrete subclass of it is [AXNodeObject], which
				417	wraps a [Node]. In turn, most AXNodeObjects are actually [AXLayoutObject]s,
				418	which wrap both a [Node] and a [LayoutObject]. Access to the LayoutObject is
sashab	b498325a	2017-05-17 01:18:46	[diff] [blame]	419	important because some elements are only in the AXObject tree depending on their
dmazzoni	2f48975	2017-02-16 03:39:16	[diff] [blame]	420	visibility, geometry, linewrapping, and so on. There are some subclasses of
				421	AXLayoutObject that implement special-case logic for specific types of Node.
sashab	b498325a	2017-05-17 01:18:46	[diff] [blame]	422	There are also other subclasses of AXObject, which are mostly used for testing.
dmazzoni	2f48975	2017-02-16 03:39:16	[diff] [blame]	423
				424	Note that not all AXLayoutObjects correspond to actual Nodes; some are synthetic
				425	layout objects which group related inline elements or similar.
				426
				427	The central class responsible for dealing with accessibility events in Blink is
				428	[AXObjectCacheImpl], which is responsible for caching the corresponding
				429	AXObjects for Nodes or LayoutObjects. This class has many methods named
				430	`handleFoo`, which are called throughout Blink to notify the AXObjectCacheImpl
				431	that it may need to update its tree. Since this class is already aware of all
				432	accessibility events in Blink, it is also responsible for relaying accessibility
				433	events from Blink to the embedding content layer.
				434
				435	## The content layer
				436
				437	The content layer lives on both sides of the renderer/browser split. The content
				438	layer translates WebAXObjects into [AXContentNodeData], which is a subclass of
				439	[ui::AXNodeData]. The ui::AXNodeData class and related classes are Chromium's
				440	cross-platform accessibility tree. The translation is implemented in
				441	[BlinkAXTreeSource]. This translation happens on the renderer side, so the
				442	ui::AXNodeData tree now needs to be sent to the browser, which is done by
				443	sending [AccessibilityHostMsg_EventParams] with the payload being serialized
				444	delta-updates to the tree, so that changes that happen on the renderer side can
				445	be reflected on the browser side.
				446
				447	On the browser side, these IPCs are received by [RenderFrameHostImpl], and then
				448	usually forwarded to [BrowserAccessibilityManager] which is responsible for:
				449
				450	1. Merging AXNodeData trees into one tree of [BrowserAccessibility] objects,
				451	by linking to other BrowserAccessibilityManagers. This is important because
				452	each page has its own accessibility tree, but each Chromium window must
				453	have only one accessibility tree, so trees from multiple pages need to be
				454	combined (possibly also with trees from Views UI).
				455	2. Dispatching outgoing accessibility events to the platform's accessibility
				456	APIs. This is done in the platform-specific subclasses of
				457	BrowserAccessibilityManager, in a method named `NotifyAccessibilityEvent`.
				458	3. Dispatching incoming accessibility actions to the appropriate recipient, via
				459	[BrowserAccessibilityDelegate]. For messages destined for a renderer,
				460	[RenderFrameHostImpl], which is a BrowserAccessibilityDelegate, is
				461	responsible for sending appropriate `AccessibilityMsg_Foo` IPCs to the
				462	renderer, where they will be received by [RenderAccessibilityImpl].
				463
				464	On Chrome OS, RenderFrameHostImpl does not route events to
				465	BrowserAccessibilityManager at all, since there is no platform screenreader
				466	outside Chromium to integrate with.
				467
				468	## Views
				469
				470	Views generates a [NativeViewAccessibility] for each View, which is used as the
				471	delegate for an [AXPlatformNode] representing that View. This part is relatively
				472	straightforward, but then the generated tree must be combined with the web
				473	accessibility tree, which is handled by BrowserAccessibilityManager.
				474
				475	## WebUI
				476
				477	Since WebUI surfaces have renderer processes as normal, WebUI accessibility goes
				478	through the blink-to-content-to-platform pipeline described above. Accessibility
				479	for WebUI is largely implemented in JavaScript in [webui-js]; these classes take
				480	care of adding ARIA attributes and so on to DOM nodes as needed.
				481
				482	## The Chrome OS layer
				483
				484	The accessibility tree is also exposed via the [chrome.automation API], which
				485	gives extension JavaScript access to the accessibility tree, events, and
				486	actions. This API is implemented in C++ by [AutomationInternalCustomBindings],
				487	which is renderer-side code, and in JavaScript by the [automation API]. The API
				488	is defined by [automation.idl], which must be kept synchronized with
				489	[ax_enums.idl].
				490
				491	[AccessibilityHostMsg_EventParams]: https://cs.chromium.org/chromium/src/content/common/accessibility_messages.h?sq=package:chromium&l=75
				492	[AutomationInternalCustomBindings]: https://cs.chromium.org/chromium/src/chrome/renderer/extensions/automation_internal_custom_bindings.h
				493	[AXContentNodeData]: https://cs.chromium.org/chromium/src/content/common/ax_content_node_data.h
				494	[AXLayoutObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.h
				495	[AXNodeObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXNodeObject.h
sashab	0cd996b	2017-05-17 07:31:46	[diff] [blame]	496	[AXObjectImpl]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXObjectImpl.h
dmazzoni	2f48975	2017-02-16 03:39:16	[diff] [blame]	497	[AXObjectCacheImpl]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/accessibility/AXObjectCacheImpl.h
				498	[AXPlatformNode]: https://cs.chromium.org/chromium/src/ui/accessibility/platform/ax_platform_node.h
				499	[AXTreeSerializer]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_tree_serializer.h
				500	[BlinkAXTreeSource]: https://cs.chromium.org/chromium/src/content/renderer/accessibility/blink_ax_tree_source.h
				501	[BrowserAccessibility]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility.h
				502	[BrowserAccessibilityDelegate]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility_manager.h?sq=package:chromium&l=64
				503	[BrowserAccessibilityManager]: https://cs.chromium.org/chromium/src/content/browser/accessibility/browser_accessibility_manager.h
				504	[LayoutObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutObject.h
				505	[NativeViewAccessibility]: https://cs.chromium.org/chromium/src/ui/views/accessibility/native_view_accessibility.h
				506	[Node]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/dom/Node.h
				507	[RenderAccessibilityImpl]: https://cs.chromium.org/chromium/src/content/renderer/accessibility/render_accessibility_impl.h
				508	[RenderFrameHostImpl]: https://cs.chromium.org/chromium/src/content/browser/frame_host/render_frame_host_impl.h
				509	[ui::AXNodeData]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_node_data.h
				510	[WebAXObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/public/web/WebAXObject.h
				511	[automation API]: https://cs.chromium.org/chromium/src/chrome/renderer/resources/extensions/automation
				512	[automation.idl]: https://cs.chromium.org/chromium/src/chrome/common/extensions/api/automation.idl
				513	[ax_enums.idl]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_enums.idl
				514	[chrome.automation API]: https://developer.chrome.com/extensions/automation
				515	[webui-js]: https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/