If you ask a layman to differentiate between "the browser" and Facebook, they might not know the difference. In a lot of ways, Facebook is a browser for a set of content types like posts, videos, business pages, etc. Same logic applies to say YouTube: it's a specialized "subbrowser" for videos. If pressed I think I'd say the only reason YouTube or Facebook aren't browsers is because they're not decentralized. They browse a fixed set of end points.

AMP project, kind of similar, it's a specialized sub browser for mobile articles. And it's like what you describe, actually: constrained, but not decentralized.

I don't think we need a new browser or web to accomplish your ask

Browsers could specialize by running "sub browser" overlay applications. If you want to a constrained subweb with markdown-only sites, then sure, why not? It's not that different than a markdown-only publishing service-- the YouTube of markdown-- except the app logic is on the client side, and it's browsing a decentralized web

our markdown website feature is built in, but it could be moved to userland as an overlay, as an app that's triggered when the user turns it on, or goes to a certain type of site. It may sound like a stretch, but out bigger picture is to invert the relationship with services to create thick client side apps, in which case an overlay app is just a particular class of app with a particular class of permissions