At the top level, the first concept to understand is the platform Accessibility API, which is an integral part of each Operating System. This is MSAA/UIA/IAccessible2 on Windows, AT-SPI/IAccessible2 on Linux, the OS X Accessibility Protocol on Mac and iOS, and so on.It may not be obvious, but these top level accessibility APIs have a direct relationship with web technology development, and are critical for the accessibility of interactive ARIA Widgets in particular.
For example, at the platform level in the Windows OS, there is a Checkbox control type. This is documented at the Microsoft Developer Network’s Checkbox Class.When a control such as this, or of any other type, is rendered as part of the Graphical User Interface (GUI), the control and all of its public properties and states, is included in the Accessibility Tree.
The Accessibility Tree is a hierarchical construct of objects that include accessible names and descriptions, plus supporting states and properties, which Assistive Technologies can interface with to enhance accessibility.
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Read full article at Source: The Accessibility Tree: A Training Guide for Advanced Web Development