The accessibility tree contains accessibility-related information for most HTML elements.
Browsers convert markup into an internal representation called the DOM tree. The DOM tree contains objects representing all the markup’s elements, attributes, and text nodes. Browsers then create an accessibility tree based on the DOM tree, which is used by platform-specific Accessibility APIs to provide a representation that can be understood by assistive technologies, such as screen readers.
There are four things in an accessibility tree object:
- How can we refer to this thing? For instance, a link with the text "Read more" will have "Read more" as its name (find more on how names are computed in the Accessible Name and Description Computation spec).
- How do we describe this thing, if we want to provide more desciption beyond the name? The description of a table could explain what kind of information the table contains.
- What kind of thing is it? For example, is it a button, a nav bar, or a list of items?
- Does it have a state? Examples include checked or unchecked for checkboxes, and collapsed or expanded for the
Additionally, the accessibility tree often contains information on what can be done with an element: a link can be followed, a text input can be typed into, etc.
While still in draft form within the Web Incubator Community Group, the Accessibility Object Model (AOM) intends to incubate APIs that make it easier to express accessibility semantics and potentially allow read access to the computed accessibility tree.