You don't even need a hack, there's built-in support for this.

https://en.wikipedia.org/wiki/Motion_JPEG

I've been here screaming "Motion JPEG EXISTS and is well supported in browsers" the entire time those gif hacks were popular. I've built a bunch of cool stuff with it.

https://github.com/donatj/mjpeg-php/blob/master/mjpeg.php

https://github.com/donatj/imgboard/blob/master/main.go

The PHP example here is an illustration of how easy it is to pull off, and the Go example is part of a JavaScript-free multiuser drawing board.

We even have proper video codecs like h264!

Sure, but most sites strip the animation out of APNGs while still allowing GIFs, which is frustrating.