Markdown, wiki markup, etc. have been around for a long time and there has never been any talk of supporting them natively in the browser.
I don't see why that would change.
I don't see why that would change.
If browsers were easier to make, someone could experiment with content negotiating for markdown and rendering it client side.
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.
I guess the best solution to that isn't browser-side .md rendering, though.