What makes CSS hard to master?

Last week, Tim Severian posted an article on what make’s CSS hard to master.

I feel we, the community, have to acknowledge that CSS is easy to get started with and hard to master. Let’s reflect on the language and find out what makes it hard. … CSS may seem simple at first, but the learning curve isn’t linear; mastering it is hard. By no means is the language perfect, but it’s amazingly efficient at what it’s designed to do. Despite its simple syntax, there’s little it can’t do.

The whole article is quite good and well worth a read.