For the curious, this is called a quine, and it's a fun challenge in every language, to write on.

http://www.madore.org/~david/computers/quine.html

Nice. Here's another: https://secretgeek.github.io/html_wysiwyg/html.html

....inspired by https://github.com/secretGeek/html_wysiwyg -- thanks!

The trick here I didn't know about is that a `<style>` element's default `display` is `none` and you can show its contents by changing that.