Imagine: the original idea used to be you'd just supply a reasonably-marked-up document and maybe some formatting suggestions and the browser/user would pick the colors and fonts and such.
To be fair, the styles they chose were almost never any better than the defaults until the early 2000s or so (and even then it was hit-or-miss) by which time the dream of the web as hyperlinked documents the user could view as they pleased was falling out of fashion in favor of making it flashy brochureware and, soon after, a tightly specified app distribution platform.
Okay, but they only did that because browsers put tools like CSS/JS in their hands to let them.
I think it was a bad choice on the browsers' part. It's a totally abdication of the browser's responsibility to render things nicely and configurably for the user. Worse, javascript is bundled malware: you're letting websites execute arbitrary code on the user's machine.
Tracking, terrible website accessibility, and various dark patterns are a direct result of this. Instead of forcing websites to conform to a document format and giving users control over how that document behaves and is rendered, browsers given those powers to websites, and unsurprisingly, this has resulted in user-hostile behaviors. And it's not all peachy for websites either: websites can do a lot more with CSS/JS, but websites also have to do a lot more.
One of my side projects is a web browser which accepts various document types (i.e. image, slide show, article, login form) and renders them. Style/behavior is configurable by the user when possible, but the site serving the document has no control over styling/behavior--they only get to serve up documents which conform to the semantics of the document type. I doubt this will ever take off, but it's been an interesting and fun project so far.