That's true, but there are various other syntax differences that mean that pasting HTML is always going to require some fixing up. For example, JSX requires all

That's true, but there are various other syntax differences that mean that pasting HTML is always going to require some fixing up. For example, JSX requires all elements to have closing tags or use the /> syntax, whereas HTML has elements like input or img where's that's not correct. That said, "class" shows up a lot more in most html than "input", so I can see the advantage of being consistent with html there.