Some places have a healthy mix. GitHub will bold and italicize the text to suggest the rendering of certain blocks, without changing font sizes. Good for reducing the need to swap between edit and preview to verify simple formatting issues as I write.

What I truly despise is when markdown blocks are parsed immediately, where the delimiters are instantly removed and the text gets formatted into a terrible WYSIWYG editor. It causes me to constantly swap mental models as I write. It’s the worst of both worlds.