Add more topics to accessiiblity documentation.

docs/accessibility.md

 # Accessibility Overview
 
-This document describes how accessibility is implemented throughout Chromium at
-a high level.
+Accessibility means ensuring that all users, including users with disabilities,
+have equal access to software. One piece of this involves basic design
+principles such as using appropriate font sizes and color contrast,
+avoiding using color to convey important information, and providing keyboard
+alternatives for anything that is normally accomplished with a pointing device.
+However, the majority of accessibility code in Chromium is concerned with

[aboxhall, 2016/11/04 17:02:07]

Rather than "the majority of accessibility code in Chromium", can we say "the
code in Chromium which is explicitly referred to as accessibility code, usually
in part of the file name or path"?

Reasoning being that e.g. all the code dealing with browser focus is arguably
"accessibility" code, as is your contrast code, but that code isn't named as
such, and the code which is named as such has a narrower focus and that focus is
what is being discussed here.

[dmazzoni, 2016/11/04 22:09:05]

Done.

+providing full access to Chromium's UI via external accessibility APIs that
+are utilized by assistive technology.
+
+Assistive technology includes:

[aboxhall, 2016/11/04 17:02:07]

How about "'Assistive technology' here refers to software or hardware which
makes use of these APIs to create an alternative interface for the user which
accommodates some specific needs, for example:" or similar?

[dmazzoni, 2016/11/04 22:09:05]

Done.

+
+* Screen readers for blind users that describe the screen using
+  synthesized speech or braille
+* Voice control applications that let you speak to the computer,
+* Switch access that lets you control the computer with a small number
+  of physical switches,
+* Magnifiers that magnify a portion of the screen, and often highlight the
+  cursor and caret for easier viewing, and
+* Assistive learning and literacy software that helps users who have a hard
+  time reading print, by highlighting and/or speaking selected text
+
+In addition, because accessibility APIs provide a convenient and universal
+way to explore and control applications, they're often used for automated
+testing scripts, and UI automation software like password managers.
+
+Web browsers play an important role in this ecosystem because they need
+to not only provide access to their own UI, but also provide access to
+all of the content of the web.
+
+Each operating system has its own native accessibility API. While the
+core APIs tend to be well-documented, it's unfortunately common for
+screen readers in particular to depend on additional undocumented or
+vendor-specific APIs in order to fully function, especially with web
+browsers, because the standard APIs are insufficient to handle the
+complexity of the web.
+
+Chromium needs to support all of these operating system and
+vendor-specific accessibility APIs in order to be usable with the full
+ecosystem of assistive technology on all platforms. Just like Chromium
+sometimes mimics the quirks and bugs of older browsers, Chromium often
+needs to mimic the quirks and bugs of other browsers' implementation
+of accessibility APIs, too.
 
 ## Concepts
 
-The three central concepts of accessibility are:
+While each operating system and vendor accessibility API is different,
+there are some concepts all of them share.
 
 1. The *tree*, which models the entire interface as a tree of objects, exposed
-   to screenreaders or other accessibility software;
-2. *Events*, which let accessibility software know that a part of the tree has
+   to assistive technology via accessibility APIs;
+2. *Events*, which let assistive technology know that a part of the tree has
    changed somehow;
-3. *Actions*, which come from accessibility software and ask the interface to
+3. *Actions*, which come from assistive technology and ask the interface to
    change.
 
-Here's an example of an accessibility tree looks like. The following HTML:
-
-```
-<select title="Select A">
-  <option value="1">Option 1</option>
-  <option value="2" selected>Option 2</option>
-  <option value="3">Option 3</option>
-</select>
-```
-
-has a generated accessibility tree like this:
-
-```
-0: AXMenuList title="Select A"
-1:   AXMenuListOption title="Option 1"
-2:   AXMenuListOption title="Option 2" selected
-3:   AXMenuListOption title="Option 3"
-```
-
-Given that accessibility tree, an example of the events generated when selecting
-"Option 1" might be:
-
-```
-AXMenuListItemUnselected 2
-AXMenuListItemSelected 1
-AXMenuListValueChanged 0
-```
-
-An example of a command used to change the selection from "Option 1" to "Option
-3" might be:
-
-```
-AccessibilityMsg_DoDefaultAction 3
-```
-
-All three concepts are handled at several layers in Chromium.
+Consider the following small HTML file:
+
+```
+<html>
+<head>
+  <title>How old are you?</title>
+</head>
+<body>
+  <label for="age">Age</label>
+  <input id="age" type="number" name="age" value="42">
+  <div>
+    <button>Back</button>
+    <button>Next</button>
+  </div>
+</body>
+</html>
+```
+
+### The Accessibility Tree and Accessibility Attributes
+
+Internally, Chromium represents the accessibility tree for that web page
+using a data structure something like this:
+
+```
+id=1 role=WebArea name="How old are you?"
+    id=2 role=Label name="Age"
+    id=3 role=TextField labelledByIds=[2] value="42"
+    id=4 role=Group
+        id=5 role=Button name="Back"
+        id=6 role=Button name="Next"
+```
+
+Note that the tree structure closely resembles the structure of the
+HTML elements, but slightly simplified. Each node in the accessibility
+tree has an ID and a role. Many have a name. The text field has a value,
+and instead of a name it has labelledByIds, which indicates that its
+accessible name comes from another node in the tree, the label node
+with id=2.
+
+On a particular platform, each node in the accessibility tree is implemented
+by an object that conforms to a particular protocol.
+
+On Windows, the root element implements the IAccessible protocol and
+if you call IAccessible::get_accRole, it returns ROLE_SYSTEM_DOCUMENT,
+and if you call IAccessible::get_accName, it returns "How old are you?".
+Other methods let you walk the tree.
+
+On macOS, the root element implements the NSAccessibility protocol and
+if you call accessibilityRole(), it returns @"AXWebArea", and if you

[Elly Fong-Jones, 2016/11/04 15:59:06]

objc methods are generally referenced like -[NSAccessibility accessibilityRole],
mimicking the call syntax

[dmazzoni, 2016/11/04 22:09:05]

Good idea

+call accessibilityLabel(), it returns "How old are you?".
+

[Elly Fong-Jones, 2016/11/04 15:59:06]

what (if anything) happens on Linux?

[dmazzoni, 2016/11/04 22:09:05]

Done.

+So while the details of the interface vary, the underlying concepts are
+similar. Both IAccessible and NSAccessibility have a concept of a role,
+but IAccessible uses a role of "document" for a web page, while NSAccessibility
+uses a role of "web area". Both IAccessible and NSAccessibility have a
+concept of the primary accessible text for a node, but IAccessible calls
+it the "name" while NSAccessibility calls it the "label".
+
+**Historical note:** The internal names of roles and attributes in
+Chrome often tend to most closely match the macOS accessibility API
+because Chromium was originally based on WebKit, where most of the
+accessibility code was written by Apple. Over time we're slowly
+migrating internal names to match what those roles and attributes are
+called in web accessibility standards, like ARIA.
+
+### Accessibility Events
+
+In Chromium's internal terminology, an Accessibility Event always represents
+communication from the app to the assistive technology, indicating that the
+accessibility tree changed in some way.
+
+As an example, if the user were to press the Tab key and the text
+field from the example above became focused, Chromium would fire a
+"focus" accessibility event that assistive technology could listen
+to. A screen reader might then announce the name and current value of
+the text field. A magnifier might zoom the screen to its bounding
+box. If the user types some text into the text field, Chromium would
+fire a "value changed" accessibility event.
+
+As with nodes in the accessibility tree, each platform has a slightly different
+API for accessibility events. On Windows we'd fire EVENT_OBJECT_FOCUS for
+a focus change, and on Mac we'd fire @"AXFocusedUIElementChanged".
+Those are pretty similar. Sometimes they're quite different - to support
+live regions (notifications that certain key parts of a web page have changed),
+on Mac we simply fire @"AXLiveRegionChanged", but on Windows we need to
+fire IA2_EVENT_TEXT_INSERTED and IA2_EVENT_TEXT_REMOVED events individually
+on each affected node within the changed region, with additional attributes
+like "container-live:polite" to indicate that the affected node was part of
+a live region. The point is just to illustrate that the concepts are similar,

[Elly Fong-Jones, 2016/11/04 15:59:06]

is this last sentence a meta-comment on the rest of the paragraph?

[dmazzoni, 2016/11/04 22:09:05]

Yes, I tried to clarify it a bit. I just didn't want someone to think that
was supposed to be a complete explanation of live region events.

+but the details of notifying software on each platform about changes can
+vary quite a bit.
+
+### Accessibility Actions
+
+Each native object that implements a platform's native accessibility API
+supports a number of actions, which are requests from the assistive
+technology to control or change the UI. This is the opposite of events,
+which are messages from Chromium to the assistive technology.
+
+For example, if the user had a voice control application running, such as
+Voice Access on Android, the user could just speak the name of one of the
+buttons on the page, like "Next". Upon recognizing that text and finding
+that it matches one of the UI elements on the page, the voice control
+app executes the action to click the button id=6 in Chromium's accessibility
+tree. Internally we call that action "do default" rather than click, since
+it represents the default action for any type of control.
+
+Other examples of actions include setting focus, changing the value of
+a control, and scrolling the page.
+
+### Parameterized attributes
+
+In addition to accessibility attributes, events, and actions, native
+accessibility APIs often have so-called "parameterized attributes".
+The most common example of this is for text - for example there may be
+a function to retrieve the bounding box for a range of text, or a
+function to retrieve the text properties (font family, font size,
+weight, etc.) at a specific character position.
+
+Parameterized attributes are particularly tricky to implement because
+of Chromium's multi-process architecture. More on this in the next section.
+
+## Chromium's multi-process architecture

[aboxhall, 2016/11/04 17:02:07]

A diagram somewhere in here would be very helpful, I think.

[dmazzoni, 2016/11/04 22:09:05]

Sounds great, but I'll save that for a future revision.

In the meantime, I added a link to the design doc on chromium.org,
which does have a diagram.

+
+Native accessibility APIs tend to have a *functional* interface, where
+Chromium implements an interface for a canonical accessible object that
+includes methods to return various attributes, walk the tree, or perform
+an action like click(), focus(), or setValue(...).
+
+In contrast, the web has a largely *declarative* interface. The shape
+of the accessibility tree is determined by the DOM tree (occasionally
+influenced by CSS), and the accessible semantics of a DOM element can
+be modified by adding ARIA attributes.
+
+One important complication is that all of these native accessibility APIs
+are *synchronous*, while Chromium is multi-process, with the contents of
+each web page living in a different process than the process that
+implements Chromium's UI and the native accessibility APIs.
+
+Chromium's multi-process architecture means that we can't implement
+accessibility APIs the same way that a single-process browser can -
+namely, by calling directly into the DOM to compute the result of each
+API call. For example, on some operating systems there might be an API
+to get the bounding box for a particular range of characters on the
+page.  In other browsers, this might be implemented by creating a DOM
+selection object and asking for its bounding box.
+
+That implementation would be impossible in Chromium because it'd require
+blocking the main thread while waiting for a response from the renderer
+process that implements that web page's DOM. (Not only is blocking the
+main thread strictly disallowed, but the latency of doing this for every
+API call makes it prohibitively slow anyway.) Instead, Chromium takes an
+approach where a representation of the entire accessibility tree is
+cached in the main process. Great care needs to be taken to ensure that
+this representation is as concise as possible.
+
+In Chromium, we build a data structure representing all of the
+information for a web page's accessibility tree, send the data
+structure from the renderer process to the main browser process, cache
+it in the main browser process, and implement native accessibility
+APIs using solely the information in that cache.
+
+As the accessibility tree changes, tree updates and accessibility events
+get sent from the renderer process to the browser process. The browser
+cache is updated atomically in the main thread, so whenever an external
+client (like assistive technology) calls an accessibility API function,
+we're always returning something from a complete and consistent snapshot
+of the accessibility tree. From time to time, the cache may lag what's
+in the renderer process by a fraction of a second.
+
+Here are some of the specific challenges faced by this approach and
+how we've addressed them.
+
+### Sparse data
+
+There are a *lot* of possible accessibility attributes for any given
+node in an accessibility tree. For example, there are more than 150
+unique accessibility API methods that Chrome implements on the Windows
+platform alone. We need to implement all of those APIs, many of which
+request rather rare or obscure attributes, but storing all possible
+attribute values in a single struct would be quite wasteful.
+
+To avoid each accessible node object containing hundreds of fields the
+data for each accessibility node is stored in a relatively compact
+data structure, ui::AXNodeData. Every AXNodeData has an integer ID, a
+role enum, and a couple of other mandatory fields, but everything else
+is stored in attribute arrays, one for each major data type.
+
+```
+struct AXNodeData {
+  int32_t id;
+  AXRole role;
+  ...
+  std::vector<std::pair<AXStringAttribute, std::string>> string_attributes;
+  std::vector<std::pair<AXIntAttribute, int32_t>> int_attributes;
+  ...
+}
+```
+
+So if a text field has a placeholder attribute, we can store
+that by adding an entry to `string_attributes` with an attribute
+of ui::AX_ATTR_PLACEHOLDER and the placeholder string as the value.
+
+### Incremental tree updates
+
+Web pages change frequently. It'd be terribly inefficient to send a
+new copy of the accessibility tree every time any part of it changes.
+However, the accessibility tree can change shape in complicated ways -
+for example, whole subtrees can be reparented dynamically.
+
+Rather than writing code to deal with every possible way the
+accessibility tree could be modified, Chromium has a general-purpose
+tree serializer class that's designed to send small incremental
+updates of a tree from one process to another. The tree serializer has
+just a few requirements:
+
+* Every node in the tree must have a unique integer ID.
+* The tree must be acyclic.
+* The tree serializer must be notified when a node's data changes.
+* The tree serializer must be notified when the list of child IDs of a
+  node changes.
+
+The tree serializer doesn't know anything about accessibility attributes.

[aboxhall, 2016/11/04 17:02:07]

Would it be dangerous to point to the specific locations in code for the
concepts discussed in these sections?

[dmazzoni, 2016/11/04 22:09:05]

My plan was to cover all of the concepts at the top *without* technical
details like source directory locations and class names, since those tend
to change much more frequently than the key underlying concepts do.

+It keeps track of the previous state of the tree, and every time the tree
+structure changes (based on notifications of a node changing or a node's
+children changing), it walks the tree and builds up an incremental tree
+update that serializes as few nodes as possible.
+
+In the other process, the Unserialization code applies the incremental
+tree update atomically.
+
+### Text bounding boxes
+
+One challenge faced by Chromium is that accessibility clients want to be
+able to query the bounding box of an arbitrary range of text - not necessarily
+just the current cursor position or selection. As discussed above, it's
+not possible to block Chromium's main browser process while waiting for this
+information from Blink, so instead we cache enough information to satisfy these
+queries in the accessibility tree.
+
+To compactly store the bounding box of every character on the page, we
+split the text into *inline text boxes*, sometimes called *text runs*.
+For example, in a typical paragraph, each line of text would be its own
+inline text box. In general, an inline text box or text run contians a
+sequence of text characters that are all oriented in the same direction,
+in a line, with the same font, size, and style.
+
+Each inline text box stores its own bounding box, and then the relative
+x-coordinate of each character in its text (assuming left-to-right).
+From that it's possible to compute the bounding box
+of any individual character.
+
+The inline text boxes are part of Chromium's internal accessibility tree.
+They're used purely internally and aren't ever exposed directly via any
+native accessibility APIs.
+
+For example, suppose that a document contains a text field with the text
+"Hello world", but the field is narrow, so "Hello" is on the first line and
+"World" is on the second line. Internally Chromium's accessibility tree
+might look like this:
+
+```
+staticText location=(8, 8) size=(38, 36) name='Hello world'
+    inlineTextBox location=(0, 0) size=(36, 18) name='Hello ' characterOffsets=12,19,23,28,36
+    inlineTextBox location=(0, 18) size=(38, 18) name='world' characterOffsets=12,20,25,29,37
+```
+
+### Scrolling, transformations, and animation
+
+Native accessibility APIs typically want the bounding box of every element in the
+tree, either in window coordinates or global screen coordinates. If we
+stored the global screen coordinates for every node, we'd be constantly
+re-serializing the whole tree every time the user scrolls or drags the
+window.
+
+Instead, we store the bounding box of each node in the accessibility tree
+relative to its *offset container*, which can be any ancestor. If no offset
+container is specified, it's assumed to be the root of the tree.
+
+In addition, any offset container can contain scroll offsets, which can be
+used to scroll the bounding boxes of anything in that subtree.
+
+Finally, any offset container can also include an arbitrary 4x4 transformation
+matrix, which can be used to represent arbitrary 3-D rotations, translations, and
+scaling, and more. The transformation matrix applies to the whole subtree.
+
+Storing coordinates this way means that any time an object scrolls, moves, or
+animates its position and scale, only the root of the scrolling or animation
+needs to post updates to the accessibility tree. Everything in the subtree
+remains valid relative to that offset container.
+
+Computing the global screen coordinates for an object in the accessibility
+tree just means walking up its ancestor chain and applying offsets and
+occasionally multiplying by a 4x4 matrix.
+
+### Site isolation / out-of-process iframes
+
+At one point in time, all of the content of a single Tab or other web view
+was contained in the same Blink process, and it was possible to serialize
+the accessibility tree for a whole frame tree in a single pass.
+
+Today the situation is a bit more complicated, as Chromium supports
+out-of-process iframes. (It also supports "browser plugins" such as
+the `<webview>` tag in Chrome packaged apps, which embeds a whole
+browser inside a browser, but for the purposes of accessibility this
+is handled the same as frames.)
+
+Rather than a mix of in-process and out-of-process frames that are handled
+differently, Chromium builds a separate independent accessibility tree
+for each frame. Each frame gets its own tree ID, and it keeps track of
+the tree ID of its parent frame (if any) and any child frames.
+
+In Chrome's main browser process, the accessibility trees for each frame
+are cached separately, and when an accessibility client (assistive
+technology) walks the accessibility tree, Chromium dynamically composes
+all of the frames into a single virtual accessibility tree on the fly,
+using those aforementioned tree IDs.
+
+The node IDs for accessibility trees only need to be unique within a
+single frame. Where necessary, separate unique IDs are used within
+Chrome's main browser process. In Chromium accessibility, a "node ID"
+always means that ID that's only unique within a frame, and a "unique ID"
+means an ID that's globally unique.
 
 ## Blink
 
 Blink constructs an accessibility tree (a hierarchy of [WebAXObject]s) from the
 page it is rendering. WebAXObject is the public API wrapper around [AXObject],
 which is the core class of Blink's accessibility tree. AXObject is an abstract
 class; the most commonly used concrete subclass of it is [AXNodeObject], which
 wraps a [Node]. In turn, most AXNodeObjects are actually [AXLayoutObject]s,
 which wrap both a [Node] and a [LayoutObject]. Access to the LayoutObject is
 important because some elements are only in the AXObject tree depending on their
@@ skipping 36 matching lines @@
    APIs. This is done in the platform-specific subclasses of
    BrowserAccessibilityManager, in a method named `NotifyAccessibilityEvent`.
 3. Dispatching incoming accessibility actions to the appropriate recipient, via
    [BrowserAccessibilityDelegate]. For messages destined for a renderer,
    [RenderFrameHostImpl], which is a BrowserAccessibilityDelegate, is
    responsible for sending appropriate `AccessibilityMsg_Foo` IPCs to the
    renderer, where they will be received by [RenderAccessibilityImpl].
 
 On Chrome OS, RenderFrameHostImpl does not route events to
 BrowserAccessibilityManager at all, since there is no platform screenreader
-outside Chrome to integrate with.
+outside Chromium to integrate with.
 
 ## Views
 
 Views generates a [NativeViewAccessibility] for each View, which is used as the
 delegate for an [AXPlatformNode] representing that View. This part is relatively
 straightforward, but then the generated tree must be combined with the web
 accessibility tree, which is handled by BrowserAccessibilityManager.
 
 ## WebUI
 
@@ skipping 29 matching lines @@
 [Node]: https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/dom/Node.h
 [RenderAccessibilityImpl]: https://cs.chromium.org/chromium/src/content/renderer/accessibility/render_accessibility_impl.h
 [RenderFrameHostImpl]: https://cs.chromium.org/chromium/src/content/browser/frame_host/render_frame_host_impl.h
 [ui::AXNodeData]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_node_data.h
 [WebAXObject]: https://cs.chromium.org/chromium/src/third_party/WebKit/public/web/WebAXObject.h
 [automation API]: https://cs.chromium.org/chromium/src/chrome/renderer/resources/extensions/automation
 [automation.idl]: https://cs.chromium.org/chromium/src/chrome/common/extensions/api/automation.idl
 [ax_enums.idl]: https://cs.chromium.org/chromium/src/ui/accessibility/ax_enums.idl
 [chrome.automation API]: https://developer.chrome.com/extensions/automation
 [webui-js]: https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/