It's not mandatory, and it would work without using it. The whole point is for forward compatibility support effect for sites which are very slow introducing these kind of changes.
replyIt's not mandatory. But if you don't use it, the browser may select a source it cannot play.
replyPossibly, but as above, a much slower problem to solve is not in the browser support, but in sites support. Browsers in practice move way faster.
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