PDF is done via an internal plugin. Standards compliant web browser doesn't have to do anything with PDF. Major browsers have internal type handler for PDF.
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.
Yeah, sending a .md for client-side rendering would allow the client to reformat it more easily based on user preferences. Then again, Safari/Firefox reader mode already do an ok job with HTML for this.
But we could go so much further than reader mode. Users should have way more control over how content is rendered. But I'm something of an extremist. I don't really consider CSS/JS part of the web.
I don't really agree about CSS/JS, but either way, I've been in plenty of situations operating informational sites that just want to serve mixed text/image without worrying too much about how it's formatted. Unfortunately there isn't such an option. Regular HTML tags are supposed to do this, but most browsers won't format those in a modern-looking way. It'd save a lot of collective time if they could.
Using plugins, you could think about Markdown, wiki markup, ...