The browser wouldn't have to comply, unless of course they are using an in browser decoder for the video. That was a big issue earlier with Firefox and H.264 support, which they skated by using OS level commands to decode the video. As far as fragmentation of DRM schemes, well that's the same now. All this adds is a common way to pass encryption keys and authentication from CDN to enduser. As Joe above noted this isn't really doing DRM per se, but just an API for allowing it.
My hope is that once stardardized this will lead to open source but difficult to break encryption, think 2048 bit PGP to secure more than just DRM media, but secure downloading of vital documents.
While DRM can be annoying especially for paying customers, I can also see the reasoning behind the HTML5 EME standards. (https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html)
Think of it this way, at least now it wouldn't matter that you run AmigaOS as long as the browser is up to date and supports HTML5, you will be able to watch Netflix, Amazon, VUDU, etc...
The specification is actually rather vague, and I could also see this as a nice way to encrypt general traffic without the need for expensive licensing or software.
If they were smart they would simply use a walled garden approach, which basically sends all traffic to a specific web page like a cafe internet sign-up.