This was just a really quick repl I made that allows live preview of HTML and CSS, so you don't have to keep pressing the run button.

I haven't done anything like syntax highlighting yet - I just made this because sometimes you need to fiddle around with styles a lot and this makes it a bit quicker.

Please upvote if you like it :)

EDIT: Following a suggestion, I have now improved it by using Ace editor.
Thank you to @TheDrone7 for the suggestions!