Challenges and Limitations
Microsoft designed the Active Accessibility object model during and after the release of Windows 95. The model is based on roles, each role representing a type of a user interface element. These roles are limited to user interface elements in common use at the time. For example, there is no text object model to help assistive technologies deal split buttons which combine multiple UI elements into one. MSAA does not attempt to represent styled text such as markup text or rich text documents. While MSAA still has the Value property, it can host only simple, non-styled text in its value. At the time, it was felt that the Microsoft Text Object Model (MS-TOM) would be more appropriate for expressing the attributes of formatted text. However, MS-TOM's complexity and limited initial adoption outside of Microsoft hampered access to rich text.
Another limitation involves navigating the object model. MSAA represents the UI as a hierarchy of accessible objects in a manner similar to Windows' Window Manager. Clients navigate from one accessible object to another using the IAccessible::accNavigate method. However, servers implemented accNavigate in unpredictable ways and often not at all. Clients, however, must be able to deal with all approaches for any MSAA server. This ambiguity means extra work for client implementers, and the complexity can contribute to problems depending on the server implementations.
Being a COM-based binary interface, IAccessible is immutable and cannot be changed without creating another interface. The result is that you cannot expose new roles, behavior or properties through the existing IAccessible-based object model. While intended to be a common subset of information about base UI elements, it was found to be difficult to extend to include information about new interaction methods.
Read more about this topic: Microsoft Active Accessibility
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