That’s what lots of sites used to do in the late 90s and early aughts in order to have fixed elements.

It was really shit. Browser navigation cues disappear, minor errors will fuck up the entire thing by navigating fixed element frames instead of contents, design flexibility disappears (even as consistent styling requires more efforts), frames don’t content-size so will clip and show scroll bars all over, debugging is absolute ass, …

And it increases resource use.

Whether it’s good enough or not it does exist and everyone can decide if it works for them.

I’d probably explore WebComponents, but wanting the height of JavaScript without JavaScript..