Use Case: Accessibility Notifications

Currently, web authors primarily alert assistive technologies to important document change events by moving focus around. For example, new content in a single-page web application is loaded so the author forces focus there. This means ATs have to heuristically determine what happened and why it's relevant to the user. iOS has several author-supplied notifications that would be useful in the context of web accessibility.

Announcement (e.g. Speak this text) and the callback AnnouncementDidFinish
LayoutChanged
ScreenChanged (single-page web apps could trigger this when loading new content)