I spent winter break in 2011 reading Zen Garden and learning CSS! It was so useful in the next seven years of my career to be able to write CSS as easily as English.
Was introduced to the site my freshman year of university ~2016. Our web professor used it as a project for getting everyone more familiar with CSS in the intro web course, and each student had to come up with a different design. I ended up making the page look like a text editor and REPL if I remember correctly. It was a lot of fun and a good learning experience!