Design and beauty matter, but I find every day websites that are utterly unusable due to some boilerplate, clueless complexity.
To test websites, I always use a barely viable client, like my nine year old iPad. Or an old Android phone.
Last night, I was looking at a US Federal government web site for seeking financial assistance with monthly internet service. You have to look up guidelines that state household income. The site didn't work without JavaScript frameworks. It was unreadable on a small Android phone with JavaScript enabled.
That surprised me a bit, because actually in general US Fed sites tend to be relatively usable. Thanks to 18F and ruthless pragmatism, perhaps.
The worst, utterly awful, things tend to be healthcare providers.
(Oh also actually my daughter tried to use her 2020-spec ChromeBook to fill out a PDF form from her university -- that was seven circles of hell. I had a hard time believing how insane that was... but that's somewhat far afield.)
If CSS and JavaScript went away, I suppose the much-needed void would require filling by legions of Kafkaesque Flash and Java apps. Or ActiveX controls.
[1]: https://kevq.uk/reality-without-css-javascript