SVG? Maybe XML/XSLT? We have also PDFs (yes it is not text). Otherwise, none in my knowledge.
Using plugins, you could think about Markdown, wiki markup, ...
Using plugins, you could think about Markdown, wiki markup, ...
Similar type handler is engaged with XML. Unless you can utilize W3C standards to implement a custom markup language using XML/XSLT and have it work across browsers without plugins.
SVG is vector graphics.
For another full markup to be even considered there would have to be one that's widely adopted and realized through plugins. Nobody is making interventions in standards to open up venues for easy implementation of custom markups when those markups are used by 0.001% of publishers.
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.