The funny part is, bog standard default HTML without any goddam css does look just about the most professional, because it is the simplest and looks like a paper

It looks the most something, but I don't think it's professional. Professional things have careful typesetting, and limited line lengths, and inline math that doesn't completely jar with the rest of the document. I think that a long document written in pure HTML might give off a vibe of "I know what I'm talking about", but I also wouldn't expect people to actually read it.

(Also my default font isn't Arial. It's some kind of serif.)