I'm much happier to see this. My concern with AMP has always been that it removes incentives for browsers to get faster, because if most of the content is using AMP there's no point in optimizing important things. Having a multi-vendor solution gives us non-Chrome browser vendors a voice at the table.

(To give a concrete example, why bother with optimizing layout-affecting animations to run off the main thread if AMP just forbids them? Such animations are useful, precisely because they affect layout; we aren't doing Web authors any favors by forbidding them instead of just making them fast.)