When those "informational" sites were normal 15 years ago, browser like Opera had user-CSS that you could just override, and had a number of presets. You could format the site to look like C64 BASIC.
The stuff you're talking about isn't about browsers its about the websites.
If you had a website that uses javascript to parse MD or any other markup, spit it out as trivial HTML with light DOM, client-side formatting can do everything you want.
The problem is that modern websites use patterns that workaround users' capability to customize the presentation of the website. They do not want you to look at their site the way you want.
Browsers can reformat clean HTML easily in theory, but I mean the defaults aren't nice, and most users aren't changing them. You have to use CSS to make a site look good by default.
I guess the best solution to that isn't browser-side .md rendering, though.
The stuff you're talking about isn't about browsers its about the websites.
If you had a website that uses javascript to parse MD or any other markup, spit it out as trivial HTML with light DOM, client-side formatting can do everything you want.
The problem is that modern websites use patterns that workaround users' capability to customize the presentation of the website. They do not want you to look at their site the way you want.