Accessible Mozilla: Tech overview of Firefox 23

Here's a list of core accessibility improvements in oncoming Firefox 23 (will be available as beta 24 June).

ARIA

A non standard extension of ARIA was introduced. Now Firefox recognize a "key" ARIA role which is mapped to push button role on IAccessible2/ATK layer. It is supposed to be used for on-screen keyboards and dialpads. AT can use  xml-roles object attribute to pick up that role.

HTML

* A major change of this release is we don't longer create a flat hierarchy for HTML selects (see bug). Say you have a HTML select having optgroup elements like

  <select>
    <option>item
    <optgroup label="group">
      <option>subitem
    </optgroup>
  </select>

then we used to have an accessible hierarchy like:

  combobox - HTML select
   option - HTML option (item)
   heading - HTML optgroup
   option- HTML option (subitem)

Firefox 23 exposes the following hierarchy:

  combobox - HTML select
   option - HTML option (item)
   grouping - HTML optgroup
     option - HTML option (subitem)


* Another change is HTML5 input@type=range is now accessible. It's exposed as a slider role and implements the value interface, i.e. you can obtain min, max and current values and step. Also it fires value change events.

Nasty bugs

A bug where JAWS lost the virtual buffer was fixed. The patch was back ported to Firefox 22 and Firefox 21.

Another bug was that we lost inactive document occasionally. So if the user didn't interact with the document and the document wasn't self updated via JavaScript then it could be not accessible for screen readers (see bug).

ISimpleDOM support

Yet another ISimpleDOM interface (ISimpleDOMText) was tear offed as continuation of this work.

Other bug fixes

* A selection bug where we wrongly claimed that unfocused text area has a selection was fixed.

* We don't prune children anymore for number of roles like text entry, menu items and etc if they have complex trees. In other words, if for example a text entry has anything else than a plain text then you will see that in hierarchy (see bug). So if you do

  <div role="textbox"><span role="button"></span></div>

then you will get a button inside the text entry.

IAccessible2. New features and performance.

After months of long discussions the IAccessible2 group released 1.3 version. It was probably most disputable release after the initial shipping. A number of assistive technology vendors requested a number of changes for better integration with the browser. So it took a time to find a common multiplier for all ideas.

So updated IAccessible2 API require some engineering love and thus it will be implemented in Firefox by parts. It'd be helpful to know which features are most crucial and needed for you as AT developer so we can give them a priority. To let us know please leave a comment in the bug or just ping me saying that. Alternatively you can vote for or comment right into the interesting bug (see sections below).

Roles

IA2_ROLE_COMPLEMENTARY_CONTENT is supposed for HTML5 aside element and WAI ARIA complementray role (see Mozilla bug).

States

A new IA2_STATE_PINNED state will be exposed on pinned tabs in Firefox UI (see Mozilla bug).

Technically speaking IA2_STATE_CHECKABLE was introduced in place of hacks used to workaround a lack of this state. For example Firefox exposes checkable object attribute (IAccessible2 hack) and STATE_SYSTEM_MARQUEED (pure MSAA approach). See Mozilla bug.

Relations

There is a bunch of new relations introduced to simplify a navigation through the hierarchy. Now you can quickly move to

• nearest containing document, instance of IAccessibleDocument (IA2_RELATION_CONTAINING_DOCUMENT)
• a containing tab pane, instance of IAccessible2_2 (IA2_RELATION_CONTAINING_TAB_PANE)
• a containing window document, instance of IAccessibleDocument (IA2_RELATION_CONTAINING_WINDOW)
• an application accessible, instance of IAccessibleApplication (IA2_RELATION_CONTAINING_APPLICATION)

Note, IServiceProvider is an alternative to described relations (strictly speaking a subsest of described relations). See Mozilla bug.

Also now you can get a next and previous elements in tab order (IA2_RELATION_NEXT_TABBABLE and IA2_RELATION_PREVIOUS_TABBABLE). It's useful for AT that implement own in-page navigation (see Mozilla bug).

New IA2_RELATION_NODE_PARENT_OF is a dual relation to IA2_RELATION_NODE_CHILD_OF. It's useful to get all logical children of the accessible (as you know a logical parent-child relation may be different from parent-child relation defined by accessible tree). Example is aria-owns (like aria-owns tree) and flat structured hierarchies like XUL tree (used for message list in Thunderbird) or ARIA tree where the hierarchy is supplied by aria-level like:

  <ul role="tree">
    <li role="treeitem" aria-level="1">Item 1</li>
    <li role="treeitem" aria-level="2">Item 1.1</li>
  </ul>

See Mozilla bug.

 

IAccessibleDocument

IAccessibleDocument is a new interface that allows you to get an anchor target, i.e. an element that the page was scrolled to after the page was loaded. Also an object of this interface is returned by relations like IA2_RELATION_CONTAINING_DOCUMENT and IA2_RELATION_CONTAINING_WINDOW. See Mozilla bug.

 

Manage the media content

The idea was to manage the media content by accessible actions (IA2 group got parsimonious for a new interface so the existing mechanism was reused).

If a screen reader starts, say, a subtitled video then it invokes IA2_ACTION_OPEN action. The browsers plays a portion of subtitled video and stops it until a screen reader invokes IA2_ACTION_COMPLETE action. This action is used to notify the browser that screen reader consumed the portion of played video subtitles, for example, it finished reading of them. If screen reader is no longer interesting to manage the media playback then it invokes IA2_ACTION_CLOSE action.

If more than one AT tries to manage the same media (i.e. AT that invoked 'open' action) then the browser holds on the video playback until all AT invoke 'complete' action. See Mozilla bug.

Performance

After IAccessible2 implementation by Firefox and AT it got clear that IAccessible2 API can be improved in certain ways to make the intercommunication between the browser and AT faster. So number of changes was introduced for performance matter.

Object attribute by name

Now you can obtain object attributes by name rather than get them all together and then parse the string (see Mozilla bug).

Relation by type

Now you have a method to get relation targets by type. You don't need to get all relations when you need a single one (see Mozilla bug).

Accessible containing the caret

You have a fast way to get a text accessible containing a caret in the subtree and the caret offset in  the text accessible. Previously the algorithm to find an accessible containing the caret failed on some cases. The idea of that approach was that a screen reader should crawl down the hierarchy asking for caret position until it reaches a deepest text accessible having the caret. It failed if you meet non text accessible on the way. As a workaround the screen readers navigated children. While the workaround worked well but it was really slow (see Mozilla bug).

Text attributes

The change is similar to object attributes. A light alternative to the method used to get all text attributes was added.  Now you can specify a string filter and get attributes you need (see Mozilla bug).

Get all hyperlinks

Usually AT needs all hyperlinks at once. Getting them one by one is unperformant (especially for out of process AT) and requires a cache implementation on the browser side. So a new method to get all hyperlinks has been introduced (see Mozilla bug).

Accessible Mozilla: Tech overview of Firefox 22

Firefox 22 reached beta status (it will be released June 24). It's time to list accessibility improvements we made for this version.

ARIA

ARIA role="note" doesn't allow name from subtree (bug) anymore. The bug caused JAWS, for example, to announce role="note" content twice.

HTML

* HTML radio group position doesn't count hidden radio elements (bug). So if the page contains hidden input@type="radio" then a screen reader doesn't take them into account announcing the number of radios.

* HTML input@type="file" changed its hierachy. Now it contains a push button and a label. Be careful if you have dependences on this hierarchy (see bug).

* HTML5 header and footer has changed their mapping according to HTML spec:

footer element that is not a descendant of an article or section element. contentinfo role;

header element that is not a descendant of an article or section element.  banner role.

XUL

XUL label@value element now implements text interface (partially). You can obtain a text between offsets but you can't get it by words for example (refer to bug). XUL label is used wide in Firefox user interface (for example, in Options dialog). Let us know if you have problems with new implementation.

ATK

RELATION_NODE_PARENT_OF has been implemented. It's exposed for aria-owns markup and XUL trees (used in Thunderbird and Firefox bookmarks).

Text interface

As I wrote before we started text interface reimplementation. Firefox 22 got improved getTextAt and getTextBefore offsets at word boundaries. Note, in case of getTextAt we had to mimic WebKit behavior rather than follow the ATK spec to keep Orca working.

Attention. It might be important

* Document load event may be fired a bit later than we used to do that, it will be fired right after all events contained in the queue at the time when document got loaded (see bug).

* IServiceProvider interface is implemented as a tear off (bug).

Firefox preferences for screen readers

I realized that nobody yet shared that new user preferences in Firefox were introduced for screen readers so I do it here.

There are couple preferences to make screen readers support smoother. And all of them - you might be not surprised - are Adobe Flash related. Flash is headache for both assistive technology vendors and for Firefox developers because flash is tightly related with hangs, crashes and security problems. Unfortunately we need to balance between usability and stability when dealing with Flash so we provide a bunch of preferences to let the user make a choice.

No plugin sandboxing for JAWS

You probably heard or even encountered yourself that JAWS started to hang occasionally in Firefox 14. It was a regression from bug where OOPP (out of process plugins) option was forced on by default on Windows Vista and later. Since it was done on security proposes then we had to choose between making Firefox unstable for JAWS users and keeping JAWS working. So we let users to decide and introduced a preference

  dom.ipc.plugins.enabled.a11y

The preference is not public and you should add it manually via about:config. For that you need to type about:config into address bar and then choose 'New' -> 'Boolean' items from the context menu. As far as I know you don't need any actions on Flash side because Adobe turns sandboxing off as long as Firefox's one is off.

AFAIK JAWS fixed the issue on their side: they started to deal with flash content as separate document. It is definitely different user experience but it must be better than security hazard. So we have an open bug to remove this pref. If you have a reason why we shouldn't do this then please comment into the bug.

Note, the preference was introduced in Firefox 18 and it has an affect only if JAWS is running.

Delayed plugin creation

We had another report about hanging screen readers (Windows Eyes and JAWS) on web pages with Flash. To workaround the problem we introduced a new boolean preference:

  accessibility.delay_plugins

This pref makes the browser to delay a plugin accessible creation. As a rule it saves from hangs. There's accompanying numeric preference used to specify a value of the delay in seconds:

  accessibility.delay_plugin_time

By default the value is 10 seconds. This means you can't interact with Flash content during 10 secs after the plugin was loaded on the web page. It's definitely bad user experience but it's still an option to keep screen readers working.

You should be aware that user entered value of accessibility.delay_plugins preference is ignored as long as JAWS or Windows Eyes screen reader is detected. In this case a plugin accessible creation is delayed automatically.

This feature was introduced in Firefox 22.

UIA text vs ATK / IAccessible2

After IAccessible2 comparison to ATK text it'd be good to get a quick look at ATK / IAccessible2 APIs and UIA text. Firefox doesn't have UIA implementation yet and there's no nearest plans about it but it's worth to compare these API because one day UIA text might become a good alternative to IAccessible2 I think.

UIA text is closer to user actions since UIA has concept of range which can be moved like a cursor through the web page. You can move it (and extend it) by characters, words, lines the same way as the user would do. And then you can get a text the range is spanned to. This means you won't ever run into restrictions of accessible tree and embedded characters as you probably do in case of IAccesisble or ATK APIs. A couple examples might be good to explain what I mean.

Example #1

If accessible tree is DOM based i.e. it's close to DOM hierarchy then ATK / IA2 text interface implementation might be tricky. For example, the following HTML paragraph

<p>hello
  <aside style="position:absolute;top:0px;left:0px;">meine freund</aside>
my friend</p>

can have the accessible tree

paragraph (html:p)
  text leaf ('hello' text)
  section (html:aside)
    text leaf ('meine freund' text)
  text leaf ('my friend' text)

If the browser is not smart enough then it doesn't remove embedded character designating html:aside element from the text of parent html:p element. In other words if the paragraph text is "hello*my friend" (where * is an embedded character for html:aside) then a screen reader have to to deal with it and it should ignore out of flow content somehow. If the screen reader is not smart enough to ignore that then it will move the user through "meine freund" text when the user moves through the paragraph text.

Example #2

In case of Firefox implementation which tends to use embedded characters for everything you can observe another kind of weird behavior. Screen reader must be smart to move by words, etc because embedded characters are used for inline objects like anchors. For instance if you have

<p>hel<a>lo</a></p>

then screen reader must juggle offsets to detect that this paragraph technically consists of one word. You can get into similar troubles when an anchor is spanned through multiple lines. So if you move by lines then the end offset pointing after embedded character never says to you whether a line end is in the middle or after the embedded object. You need to look into embedded object to detect that. It makes a screen reader logic not performant and not trivial.

A summary 

In short UIA lets you to move through the page in correspondence to web page layout while ATK and IAccessible2 allow you to move in correspondence to the accessible tree. Sometimes it makes a difference. 

So accessible tree dependent approach makes the text implementation not trivial on certain platforms (granted, on Gecko). I'm sure that everything (or mostly everything) can be implemented right on the browser side or can be workarounded by screen readers but implementation in either case must be not seamless. Note, somebody told me that Webkit has a nice ATK text implementation (or nicer than Gecko's one? I don't recall). So it's an attestation it's doable, just can be not easy.

I should notice we didn't prototyped anything yet. Of course before making any judgements (did I?) we need to implement it and screen readers should adopt it. And only after that I have a right to say whether it was so good as it looks. On the other hand ATK appeared years ago, IAccessible2 just adopted and simplified ATK ideas so that Microsoft had enough time after MSAA to invent something nice. So I'm ready to believe they did it.

ATK text pitfalls

As soon as I ensured myself I've got a good understanding of ATK text they put me back into reality. One more time I must admit myself that ATK text is unknowable like the universe. Seriously, shortly after I started the work on fixing ATK text bugs in Firefox then Orca, a Linux screen reader, suddenly felt bad. I've been suggested to compare Firefox and GEdit to see if there's a difference in implementations. So did I and then I realized that results depend on whence you start the ATK spec reading (btw, GEdit implementation doesn't always follow the spec). If you read the spec from beginning (a first sentence) then you get one result. If you read it from the end (a second sentence) then you might conclude that a different result is expected. I filed a bug against ATK. But let's read it again together, I might be missing something.

Let's consider an example: "a funny word".

* atk_text_get_text_at_offset for BOUNDARY_WORD_END

The returned string is from the word end before the offset to the word end at or after the offset.

I think you will agree that there's no word end *before* 0 offset so it can be treated as an author error. ATK doesn't say how error values should be handled so I guess any reasonable return value is allowed. Firefox returned a ('', 0, 0) triplet and that confused Orca.

Read the spec next:

The returned string will contain the word at the offset if the offset is inside a word.

This means we should return a ('a', 0, 1) triple because 0 offset is inside 'a' word (btw, that's what GEdit did).


* atk_text_get_text_at_offset for BOUNDARY_WORD_START

It is a dual problem to the issue above for the offset equal to a text length. Spec says:

The returned string is from the word start at or before the offset to the word start after the offset.

and

The returned string will contain the word at the offset if the offset is inside a word.

There's no word start after the offset but the same time the offset is inside 'word' word. Reading next.


* atk_text_get_text_after_offset for BOUNDARY_WORD_END

The returned string is from the word end at or after the offset to the next work end.

It might be not evident but 0 offset is a word end offset. A proof by contradiction. If 0 offset is not the end offset then

get_text_at_offset(0, BOUNDARY_WORD_END)

in case of single word (like 'word') should return an empty text. But this contradicts to get_text_at_offset method name semantic and Orca expectations (see the case above). Therefore 0 offset is a word end.

Then it means that the method at 0 offset should return the first word ('a' in our example). But the second sentence says that it must be a second word ('funny' in our case).

The returned string will contain the word after the offset if the offset is inside a word.

* atk_text_get_text_before_offset for BOUNDARY_WORD_START.

This is a dual problem to get_text_after_offset (word end boundary) case. Let's take an offset equal to text length.

The returned string is from the word start before the word start before or at the offset to the word start before or at the offset.

Text length offset is a word start offset. A proof is by analogy (see above). That means that 3d word is expected ('text' in our case). However the second sentence says that it should be a 2nd word ('funny' in our case):

The returned string will contain the word before the offset if the offset is inside a word.

IAccessible2 text vs ATK text

We started our accessible text rework in Firefox. It's time to revisit our IAccessible2 text implementation and compare it to ATK text.

Both IAccessible2 and ATK allows you to navigate the text by characters, words and lines. To do so they both provide these three methods:

• get text *at* offset
• get text *after* offset
• get text *before* offset

It makes clear that these methods are used to get the text at, after or before offsets. It's true in IAccessible2 world but it's no so univocal in case of ATK. ATK provides bunch of tricky boundary types that may change things. Below I will example the difference.

A difference #1. IAccessible2 getTextAtOffset may return nothing when you asked for a word. This happens when the given offset is between words (see the spec).

If the index is valid, but no suitable word (or other text type) is found, a NULL pointer is returned.

ATK always returns a word, no matter where you are because it requires to return the text between two word start or word end offsets (check out the spec).

A difference #2. IAccessible2 getTextAfter/BeforeOffset always return the requested lexem after/before the given offset.

Returns the substring of the specified text type that is located after/before the given character and does not include it. The result of this method should be same as a result for IAccessibleText::textAtOffset with a suitably increased/decreased index value.

ATK get_text_after/before_offset methods may return a lexem at the offset under certain circumstances. For example here's the case of word end boundary in get_text_after_offset method:

If the boundary_type is ATK_TEXT_BOUNDARY_WORD_END the returned string is from the word end at or after the offset to the next work end.

In short ATK and IAccessible2 supply two resembling but different concepts of text traversal. ATK allows you to move through the whole text by any method. For example, text_get_text_at_offset works nice if you pass the end offset you obtained at previous call. In IAccessible2 word you need to use a couple consisting of getTextAtOffset and getTextBefore/AfterOffset to move backward/forward. I'd say IAccessible2 text is simplified (a human friendly) version of ATK text. I won't object if somebody says that ATK looks more powerful. But the same time I'm not sure screen readers need this power. 

Anyway IAccessible2 text  looked so close to ATK and thus originally we mapped IAccessible2 boundary constants into ATK constants and we ended up with shared logic between these APIs. It was done several years ago and we never revisited this code.

Now we are going to change. IAccessible2 consumers please keep the track of our progress to catch regressions early.