I've been hiring a JS engineer at Pabio (YC S21) and I like to have an hour-long discussion with applicants instead of a whiteboard/live coding challenge as part of the process. One of the questions I ask is "Explain the first ten or so lines of the Twitter source code to me" and we go through the HTML source code of twitter.com line-by-line. It's always fun and Chris from CSS Tricks liked the idea so I wrote a blog post!