I tried going that way. Authoring HTML by hand is miserable, it totally breaks up your flow of writing. Then to make matters worse, reading HTML raw text vs reading in an editor is a painful experience as well.

I feel like if you wanted to write notes in HTML, you would need to do it from within some WYSIWYG editor environment that felt like plain text most of the time but easily let you drop into metadata, links, tables, etc.