[hsivonen]

No, EME is an API that the browser exposes to site-supplied JavaScript code. The JavaScript code uses EME to talk with a Content Decryption Module (CDM) whose exact characteristics (including the protocol you talk with it; EME sees byte buffers) is unspecified and out of the scope of the EME spec. The interface between the browser and the CDM is not being standardized.

The CDM is basically a new kind of proprietary component that exposes a CDM-specific protocol to the Web and that is annexed to the side of the browser. You should not assume the distribution model to be the same as with traditional plug-ins: E.g. in the case of Chrome OS, the CDM comes from Google itself.