The web today: as a platform for user interfaces and general-purpose content display, is far, far removed from HTML's "intended use case" as a means of sharing simple documents.

Like it or not, but HTML+CSS is being used for all kinds of UI - and right now you absolutely need JS to support state-transitions in HTML, even if those states arguably shouldn't need any script because the same logic can be expressed without using a Turing-complete programming language.