It appears that I hadn’t provided all the information earlier…

So here it is again:

Summary:
Media Capabilities allow for web sites to better determine what content to 
serve to the end user.
Currently a media element offers the canPlayType method 
(https://html.spec.whatwg.org/multipage/media.html#dom-navigator-canplaytype-dev)
 to determine if a container/codec can be used. But the answer is limited as a 
maybe/probably type answer.

It gives no ability to determine if a particular resolution can be played 
well/smoothly enough or be done in a power efficient manner (e.g. will it be 
hardware accelerated).

This has been a particular problem with sites such as YouTube that serves VP9 
under all circumstances even if the user agent won't play it well (VP9 is 
mostly done via software decoding and is CPU itensive). This has forced us to 
indiscriminately disable VP9 altogether).
For YouTube to know that VP9 could be used for low resolution but not high-def 
ones would allow them to select the right codec from the start.

Chrome has shipped it a while ago now and talking to several partners 
(including YouTube, Netflix, Facebook etc) , Media Capabilities support has 
been the number one request.

Bug: This issue is tracked in bugzilla 1409664  
(https://bugzilla.mozilla.org/show_bug.cgi?id=1409664)

Link to standard: The proposed spec is available at 
https://wicg.github.io/media-capabilities/
Platform coverage: It will be available for all platform, and exposed to all 
sites including insecure (http)
Estimated or target release: 63
Preference behind which this will be implemented: the feature is controllable 
via media.media-capabilities.enabled
Is this feature enabled by default in sandboxed iframes? If not, is there a 
proposed sandbox flag to enable it? If allowed, does it preserve the current 
invariants in terms of what sandboxed iframes can do?
DevTools bug: No particular requirements for additional devtools
Do other browser engines implement this? Chrome has shipped this since late 2017
web-platform-tests: Phttp://w3c-test.org/media-capabilities/

We do not enable the Screen Media-Capabilities extension due spec issues (in 
particular https://github.com/WICG/media-capabilities/issues/89), additionally, 
we have no way at present to implement those.

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