So they created a new event so people can use JS to create "reading indicators which move as you scroll." You know what else can do that? Scroll bars. In the browser. That don't disappear. Those tell you how far you are in the document as well.
I find myself frequently jumping to the bottom of the article to see if the small scrollbar is due to the article length or a comments section.
Scroll Timeline is a CSS-only solution. And it’s kind of a misnomer, it’s really a position-based styling mechanism that can be used without any scrolling at all[1].
At minimum, this API will allow existing functionality that relies on scroll position to perform better (first by integrating with the rest of the CSS-paint pipeline, next IIUC by offloading to GPU where possible), which itself is welcome. But it also unlocks a bunch of other styling possibilities that are either impossible without JS or rather brittle.