I don’t mean languages such as JS, CSS etc.
I mean concepts- layout engines, performance issues, the best ways to build elements on your page etc.
Here’s a stab at it:
A lot of people know (or claim to know) JS, but you can’t really make informed decisions about optimizations unless you know the environment you’re working inâaka, the DOM. Wikipedia actually has a decent article on the topic: http://en.wikipedia.org/wiki/Doc…