[kunagi7]

The average browser user doesn't know anything about CSS tweaking. Advanced users may know about it if they're curious enough or work with web technologies. People get pushed around by other reasons, mostly things like OS integration (Edge, Chrome).

> There's Vivaldi, and it too uses HTML for its interface, but the team doesn't seem to care about UI performance at all.

Thanks to that, Vivaldi also actively supports CSS tweaks like Firefox. They do indeed care about UI performance and work towards fixing it... The 3.7 update really improved its UI responsiveness. Still, it feels slower than Chrome/Brave/Firefox.