Re: OK, so how does that work?
Problem is we're not talking about Firefox here. We're talking of Chrome and Internet Explorer. They're both closed source so there is ample opportunity to encrypt / decrypt content in a way which is at least as secure as using a Flash plugin but with content which is HTML5 but with the video src pointing at some encrypted content.
Even Firefox could implement something similar if they wished given that they release binary builds which could contain stuff which is not in the source. And failing that someone could simply produce a binary extension / plugin for Firefox which bundles up a video player which supports the DRM and places it in the page. This would not be rocket science to do.