Yes, it needs to work, fully, without CSS or JS. Then, use CSS to make it look nicer, and use JS to make it behave nicer.
The total failure of most people who build professional web sites to follow standards and make them pain-free continues to baffle me, when it’s not that hard to do right and helps ensure the site is maintainable going forward, regardless of which new JS library or framework becomes popular.
This doesn't happen for me when clicking around sites like HN or my blog, both are server side rendered and I'm on a pretty flakey connection right now. Browsers solved this a long time ago with how they deal with page changes.
I read the server-side-rendered forums of DLang, and everything happens so fast, in around 200 ms, that I don't feel I am losing anything by the forums not being an SPA.
I mean, is this really a legitimate problem? Full page reloads don't bother me very much at all, and they actually come with a lot of niceties. Like URLs and the back button working normally.
For me personally, not usually. But having worked on a few different companies with embeddable widgets, yes it is. The page-based (SSG) routing is not an option there. And even in CRUDs the product folks want WYSIWYG and smooth uploads of assets. With more and more bling it's just inevitable that one will need a JS framework eventually.
But datastar (they just released v1 2 days ago) is even better.
MPA, server-rendered, but with client-side reactivity pushed from the server via SSE.
It's pretty impressive - better than HTMX imho.