I think the browser is supposed to load them by the order listed in the HTML. <video> <source src="foo.mp4"> <source src="foo.webm"> </vide

I think the browser is supposed to load them by the order listed in the HTML. <video> <source src="foo.mp4"> <source src="foo.webm"> </video> Unfortunately, I think there were bugs in older browsers, so they broke if mp4 was not listed first. So most websites list mp4 first.