A dozen imgur tabs with a single gifv each have caused me notable background CPU use in Firefox. A dozen tabs with gifs in them don't.

That doesn't really count. A dozen tabs in Firefox might also give you a rash, the solution is to probably stop using Firefox not to convert videos to gifs.

That's not a fair comparison with gif/apng if the thing can pick between webm or mp4 to get hardware acceleration and start/stop off-screen videos.

Why is it not a fair comparison? A page full of gifs in a browser gobbles bandwidth, can't be interrupted, eats lots of cpu. A page full of gifvs doesn't have any of these problems. One of these is a lousy user experience, the other one is not. I don't have to make both of these things a shitty user experience just so the comparison is 'fair'.