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For Firefox there are actually a couple of rendering add-ons, you don't like any of them?

I also write everything in md, but don't render — just styled my neovim for readability. In vim & neovim you can follow links to other local md files, web, etc. The benefits of this are instant navigation within both the folder structure and the document itself.

Also using nvim-twilight for dimming out non-focused text blocks.

There are rendering plugins as well, I just don't really need them. In my style headings are bold, different color depending on the level, # are set to be barely visible — kinda nicer than html/pdf for me personally.

Maybe there are some useful plugins for your TE/IDE?