You nailed it about the appeal of W3: It's quick to get to the point.

I can read the first para and figure out what I'll be learning. The code examples are interactive and visual, and help enormously.

MDN reads like a technical reference. Wordpress documentation is similarly structured, but it has a comments section with a lot of sample code for different use cases, which helps me figure out whether the function I'm looking at will help accomplish what I am trying to do.