>>6I'd take it one step farther for the CSS/JS bits. It should be
possible for a web browser to be written that doesn't fetch any CSS/JS/etc from a server at all - by which I mean it just doesn't follow
href tags in
<link rel="stylesheet"> tags, not that it magically predetermines what the content of an item to retrieve will be - and likewise doesn't execute any javascript from the server. On the contrary, it relies on only local CSS and JS, as if it were Userstyles and Userscripts only. It should be possible for such a hypothetical browser to navigate any page on the web just fine. Ideally, there should be naming conventions for divs (like HTML tags themselves could have been, before W3C got its head stuck even farther up its ass) so that a single master CSS file can control any normal website to the user's specification.
I remember when I actually thought web development might be a thing that didn't suck. I saw
http://www.csszengarden.com/ and thought ``This is going to take off, this is going to be big - I'm going to be able to theme any well-written page any way I like. I'm going to be able to flip a switch at 7 PM and make every webpage have a dark theme.'' And that still should be something that could happen.