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.

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jaffathecake an hour ago | parent [-]

It's not mandatory. But if you don't use it, the browser may select a source it cannot play.

	▲	shmerl an hour ago \| parent [-]
		Possibly, 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.