One of the most perplexing and befuddling things in CSS for me, for many years, was the behaviour of percentage-based heights. Sometimes, seemingly at random, setting height: 100% would have no effect at all. When I finally figured out what was going on, it was like a puzzle piece snapping into place; everything made so much more sense! In this post, I’ll share the epiphany I had, and we’ll explore some solutions.