That is indeed what I meant by "efficiently recompressed." 10 seconds of H.264 video are still a lot larger than a JPEG.

of comparable resolution maybe, but the file size is still not large for say a 640x480 video, less than a few Megabytes.

I stopped clicking imgur gifv links a looong time ago. They worked for me maybe one out of eight times. By now I have an allergic reaction just to seeing them, even if the issues may have been fixed by now.

October 2014 -> May 2016 is 1.5 years, not 2.5. It's all of 2015 and bits of 2014 and 2016.

No it started when 4chan allowed WebM

2.5 years and it still never works on mobile.