I am not entirely sure which players actually use this information, but I am led to believe that YouTube may, based on this comment on reddit: https://www.reddit.com/r/firefox/comments/cizjii/100_cpu_usage_while_playing_some_videos/evbdxkv/ Even if they do not, this is something that Mozilla could encourage players to check to ensure that playback uses an efficient decode method as a prioritized method. Bug 1400787 shows an example of Firefox performing poorly compared to other browsers where this could help as well.
Bug 1569686 Comment 2 Edit History
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I am not entirely sure which players actually use this information, but I am led to believe that YouTube may, based on this comment on reddit: https://www.reddit.com/r/firefox/comments/cizjii/100_cpu_usage_while_playing_some_videos/evbdxkv/ Even if they do not, this is something that Mozilla could encourage players to check to ensure that playback uses an efficient decode method as a prioritized method. Bug 1400787 shows an example of Firefox performing poorly compared to other browsers where this could help as well. A bad situation occurred on that same reddit thread, where YouTube served the user AV1 video on a baytrail n3540 -- it is unclear to me whether YouTube decided to send this because of the Media Capabilities API of whether there is some other heuristic in place.