Some platforms do have APIs like that. For example, on Windows, UI Automation is not tied to any particular UI framework; the UIA tree just needs to be associated with an HWND (Win32 window handle). But implementing these APIs is a lot of work. And I imagine it would be especially challenging for an immediate-mode toolkit, because an API like UI Automation is based on a retained tree of objects that the screen reader or other assistive technology can query on demand. So I figure applications are more likely to be accessible if I warn developers away from such toolkits altogether.