Earlier quoted context omitted.
Parallax. My favorite implementation is on the homepage of Campo Santo's Firewatch game: https://www.firewatchgame.com/ Not only the parallax itself, but the creativity! They used semitransparent layers so that when you scroll it looks like there are specular reflections! (the glittery specks of light)
Interesting, it's laggy even on my M1 macbook + Firefox.
Probably something wrong with the math, because if take the scrollbar and move it slowly you would notice how some layers starts to bob up and down, instead of monotonic movement.