Would that somehow lead to Hacker News or Facebook encrypting their site's source code with that DRM? I sort of doubt it. They could do that now with Flash if they wanted to, but no one thinks Flash websites are a good idea.
1) EME as currently proposed is video only, but will the W3C try to do the same for images? For text? There are certainly constituencies who would live simple turnkey DRM for those on the web. And they don't do it in Flash _precisely_ because Flash websites are not a good idea.
2) If the above happens, there is no incentive for Netflix, say, to try come up with a setup that works with open-source browsers and still satisfies the rightsholders for the content they distribute. If people complain they'll just be told the spec says browsers need to implement DRM, and if that means no open-source browsers tough luck.
What exactly do you think will prevent them from building their already-existing in-house DRM (three different systems, note!) into their own browsers?