[russellsprouts]

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.

[shmerl]

May be an add-on can allow overriding the priority. I'll take a look if it's possible.