In my text browser I came up with a way to implement these that doesn't move boxes around the page.[0] In short: treat "position: fixed" as an absolute child of the root box, and ignore "position: sticky" (mostly).

This doesn't completely eliminate sticky headers/footers (problematic when you actually want to use them), but they behave like normal elements at the start/end of the page (instead of the screen).

I initially did it this way for technical reasons, but now I kind of prefer it to what mainstream browsers do.

[0]: https://git.sr.ht/~bptato/chawan/tree/e56399f92d2323f9af95e0... (not a great explanation - it says "bottom", but like "position: absolute" it's placed at the top if only "top" is specified, etc.)