[makomk]

Actually, it's worse than that. The interop between the browser and CDM isn't standardised at all. The only thing this standardises is the browser API that websites should use to request that the browser communicate with the CDM on their behalf.

The interface between the browser and the CDM is proprietary and unspecified, so browser vendors and CDM providers have to negotiate that themselves. The format of the encrypted binary messages passed to the Javascript API is proprietary and unspecified. The API used to communicate with the license server is also proprietary and unspecified, so it doesn't even provide much interop from the media provider perspective - they still have to write a whole bunch of DRM-provider-specific code for every DRM scheme, and it looks like every browser vendor will have their own one.

Basically, it standardises just enough to give media providers the ability to claim they're using pure HTML5, without offering any more interoperability than if every browser vendor just had their own proprietary HTML5 extension for DRM. It's a PR stunt rather than a meaningful attempt at interoperability.