Re: I don't see a problem.
You can't have an open source implementation of a DRM'ed browser without it leaking the content.
Actually it is possible, just very difficult.
Which is why the standard is recommending putting the DRM piece in the CDM, not the browser.
The CDM is the Flash-equivalent binary, except way simpler. The idea is to reduce the scope of the proprietary bits to the minimum needed to support DRM. It's a compromise that is actually very open source friendly.
And whether W3C approves it or not, it's already been here for years. Have you watched HTML5 video from Amazon, Netflix, Hulu, PornHub, et al. In any browser? Then you've been using a CDM.