Most browsers let you extend a page’s css (at least via extensions)
I don’t think this would be immune to that.
Until Google realizes you're using the capability to hide advertising and disables the feature to create a "trusted client" (trusted by Google to render their ads faithfully). They've pitched the idea before as a way to force users into Chrome from the server-side.
I'll go back to shouting at clouds now... Sorry.
Some pages seem to randomize element IDs to prevent adblockers from doing their job; randomizing CSS class names wouldn't be out of the ordinary.
if anything a standard element is easier to style on your end than everyone using a custom one right?
Not really, because for a native element you're dependent of their APIs. For example the color picker will only use the system color picker, a lot of other native elements are hard to style. So custom elements are often easier to style but harder to maintain and usually bad for accessibility.
"styling on your end" means the _user_ (not the website designer/developer) deciding how controls should be styled and that's much easier if everything uses the same controls because you only need to specify it once and don't need a library to hook all possible UI frameworks that exist.
Users deciding how input controls are styled? Who exactly asked for THAT?
Heard of userstyles?
Yes, one can style them with a userstyle as always.