CSS and Javascript both. Forget legacy codebases, people seem to forget how much legacy documentation, tutorials, and Q&A is out there confusing newbies who don't have the context to know what's no longer relevant or accurate.

In answering questions from actual newbies completely new to this, I was a bit surprised at just how much I took for granted because I was around for the historical context.

I think it is more helpful to explain why thinks work the way they do.