Calling it WYSIWYG is a misnomer: this is just syntax highlighting, it just so happens that the highlighting styles agree to an extent with the final rendering. And the idea is not exactly new: https://mediawiki.org/wiki/User:Remember_the_dot/Syntax_high...

Two issues I saw:

- on my phone, the framerate noticeably drops as I scroll over the widget

- caret positioning within the textarea seems to desync against the letter positions in the highlighted div

And the whole concept severely constrains the styling choices that could be applied to the highlighted output, but I suppose that was a given.