Although I also know my way around frontend development and design as well, I'm far less skilled at it. As such this website is trying to impress in a different way:
It implements parts of the HTML parser spec to tokenize and highlight it's own source code.
You can find the full source code of the site here. It's licensed under BSD-2-Clause, so feel free to use the code as a reference to implement your own parser! If you have something interesting to share or just want to tell how the code might have helped you, make sure to send me a message :^)