Ask HN: What's the name of the website where the structure changes as I scroll?

Hey HN, My memory is a bit fuzzy, but I recall clicking on a website through HN. The website was about learning CSS or layout. As I scrolled slowly, the website layout changed. The author was promoting a book at the end of the page. I can't seem to remember the website's name, but I'm looking to buy that book. Please comment if you know the website. Thanks in advance.