According to the spec, aria-live “indicates that an element will be updated, and describes the types of updates the user agents, assistive technologies, and user can expect from the live region.” But if an element is rendered by JavaScript shortly after page load (even 25ms), or on a transition, then screenreaders will not reliably notice content added to it.

To be clear: aria-live is evidently useless for late elements.

The solution is to have two elements either outside the application root element, or immediately rendered in the application element, dedicated to both aria-live=”polite” and aria-live=”assertive”. You can populate these elements in a number of ways, but the important thing is that these elements are picked up by the screenreader almost immediately at page load.

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Thanks to Ryan Florence & Aaron Cannon for bringing this to my attention.

“Warning! There are currently no known implementations of the outline algorithm in graphical browsers or assistive technology user agents, although the algorithm is implemented in other software such as conformance checkers. Therefore the outline algorithm cannot be relied upon to convey document structure to users. Authors are advised to use heading rank (h1-h6) to convey document structure.” (HTML 5.1 nightly)

Read more about the drama here and here.