Yes, I think that it would be good, but still the specifications and implementations of WWW are bad and tend to be trying to make the company to control everything, like they say.
Web browsers could be designed much better, although to compensate for bad specifications requires some messy details. (One thing that might help is for client software to use ARIA hints. They seem to want to use them for blind users, but I think they can help improve it for everyone.)
Instead, they make them worse, trying to hide stuff from end users, trying to believe they know better than the end users, etc. This makes things difficult to control, e.g. you cannot make non-tunneling proxies for HTTPS and therefore you need to take extra energy just to decrypt and reencrypt the communications, and the WebExtensions is rather limited, and you cannot usually make browser extensions in C, and the way that some things are core vs extensions makes many things difficult to change. And then, they will add spyware and stuff, and other inefficient stuff.
A web browser designed only for advanced users, who are assumed to know what they are doing, would be a good thing to have. (And, if you do not understand something, then read the documentation; programs must have good documentation, and unfortunately many do not, and that is what makes them difficult to understand.)
> but then wouldn't that be an argument for Emacs having its own web browser?
Maybe; they can see if a better one can be made. It shouldn't be a built-in feature of Emacs, but it could be a file included with it, and maybe it might help. Variants of Chromium isn't the way to make a better one, so it must be made differently.
> If I make my own website, I should be able to control what's on it.
Yes, but you shouldn't need to add all sorts of stuff just to concede to W3C's demand, to use Unicode, badly made HTML, etc (even if other file formats would be better, trying to serve them properly just makes a mess).
You can use just plain HTML, but it has many problems, and CSS/JavaScripts just makes even more problems.
Even if you can, it does not necessarily mean, that it is a good idea.
> Is it user-hostile for me to, say, require javascript for my site to function?
It depends on the site. For documents, it shouldn't. For some types of games, it might make sense to require JavaScripts, but it should be possible to view the documentation (and the link to the documentation) even without it.