What are the mitigations to prevent sites from throwing prompts that require users to turn off auto play to continue browsing?
The current Play method on the HTMLMediaElement already provides information via the rejection handler https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/play#exceptions, that media playback was NotAllowedError - if a site is truly interested in providing fallback content, they can catch this exception and try again with the stream muted.