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.)
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