Notion does all kinds of awful stuff. It’s horribly inefficient, reimplementing uncommonly large part of the functionality of the browser, in custom, very buggy ways. A large part of its problems stem from an absurd abuse of contenteditable when viewing pages: it makes everything contenteditable, and then tries to undo what that means with JavaScript (and makes a hash of it, of course). Related, its keyboard accessibility is terrible (try navigating around!).