I'm always enjoying interactivity to make things understandable. It was the explorative way that I got into coding in the first place. Well, let's gather some great examples here:
Amit Patel's interactive visual explanations on redblobgames.com - impossible to talk about these without mentioning Red Blob Games first. All of these are great!
Luke Pacholski aka @flukeout - CSS Diner - whenever I try to remember this I search "bento css", today I have no idea why. Doesn't seem to match no more. However. Great page!
... I once came across an interactive site about Signed Distance Functions (SDF) with little code fields with broken snippets one had to fix to make an SDF rendering work. There were the typical orange lines (like in this shadertoy example) and it was divided in a couple lessons first 2D then 3D getting more complex. I'll post it here when I find it :)