Presentation is left to the reader’s client. Do you want dark mode? Get a markdown editor/reader that supports it. Do you want serif font? Again, that’s client’s choice and not part of the document.
I remember when that is how the web worked. All that markup was to define the structure of the document and the client rendered it as set by the user.
Some clients were better than others. My favourite was the default browser in OS/2 Warp, which allowed me to easily set the display characteristics of every tag. The end result was that every site looked (approximately) the same, which made browsing so much nicer, in my opinion.
Then someone decided that website creation should be part of the desktop publishing class (at least at the school I taught at). The world (wide web) has never recovered.