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What is the Accessibility Tree?

4:29 HTML 5 lesson by

Coding for accessibility? You should get familiar with the accessibility tree, a structure produced by platform Accessibility APIs running parallel to the DOM, which exposes accessibility information to assistive technologies such as screen readers. There are multiple tools for visualizing this tree; in this lesson we'll look at Chrome and Microsoft Edge. For more on Accessibility APIs, refer to this amazing article by Leonie Watson. For a how-to on setting up the Chrome Accessibility Inspector, visit bit.ly/chrome-a11y.


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Coding for accessibility? You should get familiar with the accessibility tree, a structure produced by platform Accessibility APIs running parallel to the DOM, which exposes accessibility information to assistive technologies such as screen readers. There are multiple tools for visualizing this tree; in this lesson we'll look at Chrome and Microsoft Edge. For more on Accessibility APIs, refer to this amazing article by Leonie Watson. For a how-to on setting up the Chrome Accessibility Inspector, visit bit.ly/chrome-a11y.

Hi. Today, I'm going to show you how to look at the accessibility tree, which is information exposed by a web browser through accessibility APIs to be utilized by screen readers and other assistive technologies.

It strips out information that really isn't useful to a screen reader like scripts and meta tags in the head of a document. It really just leaves the information that's useful for accessibility.

There's a number of ways that you can look at this information. The first method I'm going to show you is if we go into Chrome. I have an accessibility experiment enabled that gives me an accessibility tab in my toolbar, not to be confused with the accessibility properties, which is an extension that runs an audit.

When you go to this accessibility tab in the elements console, you can see that it exposes some accessibility information. In Chrome, the header element exposes the role of banner, because it is the global header. I think Chrome might be smart about looking at nested headers, but I'd have to go and check. This tool would help me go and find that.

I can go through each of these elements. I can see that sections implicitly have a role of region. This is going node by node. I can go and look at each element and look at its accessibility information. If I'm looking at a button, see if it's labeled or a form element, if that's labeled.

This information is part of a tree that I want to show you the rest of it. If you go to chrome://accessibility, you get every tab and its accessibility information as something you can go and look at. Obviously, these are turned off, because it would be a huge performance problem to be building and showing this tree all the time.

The tree is always there, but to actually show it to you in this text format, they turn it off. I have a bunch of tabs open elsewhere in my browser, but I'm looking for the Egghead version. If I click accessibility off, it will turn accessibility on, and then I can show the accessibility tree.

When I expand this thing, this is the not-pretty version. This is the raw dump of the accessibility tree and all of its information. There's things like AX Link, and it has a role description of link, has a title of browse. Every element on your page has accessibility information, whether or not you've thought about it before.

I want to show you another tool that's new, and it's pretty useful. It's better designed than the Chrome dump. I'm using a Microsoft Edge Windows 10 preview that I've downloaded from modern.ie. I'm using the virtual box version.

If I open a web page in Edge and I go to inspect an element, in the DOM explorer, I get this little icon that says "accessibility tree." Admittedly, I had to go looking for it, but I knew this feature was there. What it does is it will show you all of the same information as a tree. We can see we've got that global header with a banner role. It tells us that inside of it, it's got a heading.

I'm looking at my own website, marcysutton.com, which I've actually thought about some of this stuff before. However, having it presented to you in this tree format is so useful, because you can go and look at what you've added or what you've forgotten.

Each section of the page should be within a landmark, such as a header with a role of banner, or a main region, or a footer with content info. Each one of those needs a heading to describe what it's for. Then, the content inside of that needs headings.

For example, if I scroll down on my actual page, you can see I've got a YouTube video here from my last conference talk. That is an iframe that I'm pulling in from YouTube, but I manually went onto the iframe and added a title attribute of accessibility and performance on YouTube.

When that iframe gets rendered, it automatically gets titled with that title attribute. That's a nice pattern for iframes when you're pulling in videos like that. When I looked at it in the Edge accessibility tree inspector, I could see that the application, which that role is coming from YouTube, but it's being labeled with this title that I added.

Within there, there's other headings, and there's other content. There's a complementary region for the sidebar. Just like inspecting styles, this method gives us a lot more information about accessibility, so you can actually see the tree that a screen reader would navigate by.

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