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Thread: Browsers and the accessibility tree
Number of posts in this thread: 4 (In chronological order)
From: Jackson, Derek J
Date: Mon, Mar 02 2020 2:01PM
Subject: Browsers and the accessibility tree
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Hello,
I have a technical question about how a browser creates an accessibility tree. I am having a hard time finding anything that provides many details about how this works aside from it being a parallel structure to the DOM. I have found some references to the Accessibility tree getting built from the render tree, or based on the DOM, or as part of the layout/reflow process right before painting. Does anyone know of a resource they could point me to about the technical process that is used to build the accessibility tree in a browser? Or does anyone know off hand at what stage in the rendering engine the accessibility tree is created and from what model? Any help is greatly appreciated.
Thank you,
Derek
â
Derek Jackson
Digital Accessibility Developer | Digital Accessibility Services
Harvard University Information Technology
1430 Massachusetts Ave, 4th Floor
Cambridge, MA 02138
he/him/his
From: Bateman, Jill
Date: Mon, Mar 02 2020 2:26PM
Subject: Re: Browsers and the accessibility tree
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Hi Derek,
This video provided some big ah-ha moments for me for understanding the accessibility tree - I'll start you in about 5 minutes, but the whole thing is really good: https://youtu.be/z8xUCzToff8?t=289 - probably not everything you're asking here, but a good place to start.
Best regards,
Jill
OHIO UNIVERSITY
OIT Office of Information Security & Accessibility
Jill Bateman, MA '13
Digital Accessibility Coordinator
WUSOC 275
1 Ohio University Drive
Athens, OH 45701-2979
T: 740.597.5455
= EMAIL ADDRESS REMOVED =
www.ohio.edu/oit/accessibility<;https://www.ohio.edu/oit/accessibility>
<http://www.ohio.edu/oit/web>;
From: WebAIM-Forum < = EMAIL ADDRESS REMOVED = > on behalf of Jackson, Derek J < = EMAIL ADDRESS REMOVED = >
Sent: Monday, March 2, 2020 4:01 PM
To: = EMAIL ADDRESS REMOVED = < = EMAIL ADDRESS REMOVED = >
Subject: [WebAIM] Browsers and the accessibility tree
Hello,
I have a technical question about how a browser creates an accessibility tree. I am having a hard time finding anything that provides many details about how this works aside from it being a parallel structure to the DOM. I have found some references to the Accessibility tree getting built from the render tree, or based on the DOM, or as part of the layout/reflow process right before painting. Does anyone know of a resource they could point me to about the technical process that is used to build the accessibility tree in a browser? Or does anyone know off hand at what stage in the rendering engine the accessibility tree is created and from what model? Any help is greatly appreciated.
Thank you,
Derek
Derek Jackson
Digital Accessibility Developer | Digital Accessibility Services
Harvard University Information Technology
1430 Massachusetts Ave, 4th Floor
Cambridge, MA 02138
he/him/his
From: Mark Rogers
Date: Mon, Mar 02 2020 3:01PM
Subject: Re: Browsers and the accessibility tree
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Hi Derek
It varies quite a lot depending on the browser, AT and accessibility API.
Safari calculates a11y tree relations from the DOM when the platform API is called - there's no actual tree stored in memory. For example, a call to IAccessible2::get_Parent calls AccessibilityObject::parentObjectUnignored which follows DOM parent links to find the first ancestor exposed as an accessible object:
https://github.com/WebKit/webkit/blob/9029c43e695bf886fffb15eec951f0605e34509b/Source/WebCore/accessibility/AccessibilityObject.cpp#L466
I think the Chromium a111y layer is mostly inherited from WebKit but has diverged since the blink/webkit fork.
Firefox creates a completely parallel a11y tree to the DOM, and only does this when a screen reader is connected to the browser. The code for the generic a11y tree is here:
https://dxr.mozilla.org/mozilla-central/source/accessible/generic/
It looks like Firefox creates the tree on page load and keeps it updated by listening to DOM changes:
https://dxr.mozilla.org/mozilla-central/source/accessible/generic/DocAccessible.cpp#1536
With NVDA and IE11 the a11y tree is actually calculated by NVDA using a mashup of MSAA and DOM calls (MSAA was the first a11y API so is more primitive than later APIs). I think it's calculated on the load event, then re-recalculated on events like click (and is prone to getting out of sync with the DOM).
Does that help?
Best Regards
Mark
--
Mark Rogers - = EMAIL ADDRESS REMOVED =
PowerMapper Software Ltd - www.powermapper.com
Registered in Scotland No 362274 Quartermile 2 Edinburgh EH3 9GL
On 02/03/2020, 21:02, "WebAIM-Forum on behalf of Jackson, Derek J" < = EMAIL ADDRESS REMOVED = on behalf of = EMAIL ADDRESS REMOVED = > wrote:
Hello,
I have a technical question about how a browser creates an accessibility tree. I am having a hard time finding anything that provides many details about how this works aside from it being a parallel structure to the DOM. I have found some references to the Accessibility tree getting built from the render tree, or based on the DOM, or as part of the layout/reflow process right before painting. Does anyone know of a resource they could point me to about the technical process that is used to build the accessibility tree in a browser? Or does anyone know off hand at what stage in the rendering engine the accessibility tree is created and from what model? Any help is greatly appreciated.
Thank you,
Derek
â
Derek Jackson
Digital Accessibility Developer | Digital Accessibility Services
Harvard University Information Technology
1430 Massachusetts Ave, 4th Floor
Cambridge, MA 02138
he/him/his
From: Jackson, Derek J
Date: Tue, Mar 03 2020 7:02AM
Subject: Re: Browsers and the accessibility tree
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Thank you! Both of these replies help a lot! The Rob Dodson a11y cast is a great overview, and the look at the code is exactly the technical detail I needed. I really appreciate the help!
All the Best,
Derek
â
Derek Jackson
Digital Accessibility Developer | Digital Accessibility Services
Harvard University Information Technology
1430 Massachusetts Ave, 4th Floor
Cambridge, MA 02138
he/him/his
On 3/2/20, 5:01 PM, "WebAIM-Forum on behalf of Mark Rogers" < = EMAIL ADDRESS REMOVED = on behalf of = EMAIL ADDRESS REMOVED = > wrote:
Hi Derek
It varies quite a lot depending on the browser, AT and accessibility API.
Safari calculates a11y tree relations from the DOM when the platform API is called - there's no actual tree stored in memory. For example, a call to IAccessible2::get_Parent calls AccessibilityObject::parentObjectUnignored which follows DOM parent links to find the first ancestor exposed as an accessible object:
https://github.com/WebKit/webkit/blob/9029c43e695bf886fffb15eec951f0605e34509b/Source/WebCore/accessibility/AccessibilityObject.cpp#L466
I think the Chromium a111y layer is mostly inherited from WebKit but has diverged since the blink/webkit fork.
Firefox creates a completely parallel a11y tree to the DOM, and only does this when a screen reader is connected to the browser. The code for the generic a11y tree is here:
https://dxr.mozilla.org/mozilla-central/source/accessible/generic/
It looks like Firefox creates the tree on page load and keeps it updated by listening to DOM changes:
https://dxr.mozilla.org/mozilla-central/source/accessible/generic/DocAccessible.cpp#1536
With NVDA and IE11 the a11y tree is actually calculated by NVDA using a mashup of MSAA and DOM calls (MSAA was the first a11y API so is more primitive than later APIs). I think it's calculated on the load event, then re-recalculated on events like click (and is prone to getting out of sync with the DOM).
Does that help?
Best Regards
Mark
--
Mark Rogers - = EMAIL ADDRESS REMOVED =
PowerMapper Software Ltd - www.powermapper.com
Registered in Scotland No 362274 Quartermile 2 Edinburgh EH3 9GL
On 02/03/2020, 21:02, "WebAIM-Forum on behalf of Jackson, Derek J" < = EMAIL ADDRESS REMOVED = on behalf of = EMAIL ADDRESS REMOVED = > wrote:
Hello,
I have a technical question about how a browser creates an accessibility tree. I am having a hard time finding anything that provides many details about how this works aside from it being a parallel structure to the DOM. I have found some references to the Accessibility tree getting built from the render tree, or based on the DOM, or as part of the layout/reflow process right before painting. Does anyone know of a resource they could point me to about the technical process that is used to build the accessibility tree in a browser? Or does anyone know off hand at what stage in the rendering engine the accessibility tree is created and from what model? Any help is greatly appreciated.
Thank you,
Derek
â
Derek Jackson
Digital Accessibility Developer | Digital Accessibility Services
Harvard University Information Technology
1430 Massachusetts Ave, 4th Floor
Cambridge, MA 02138
he/him/his