How much of that is down to browser bare-bones defaults before the page defines its own styles, IIRC bare HTML doesn't define what the client must do because at that level it's not defining a presentation.

I could see a browser making a change to have their defaults for an undefined style to something else, other colors and fonts or closer to reader mode, with an options toggle or flag to restore old behavior. I doubt many browsers would bother as unstyled pages are so rare for most of the web.

Theoretically if pure-HTML documents without CSS/JS got more common, end-users could just apply ther own 'themes' to HTML documents through their browser.

"HTML needs to require bigger CSS resets."