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 in—aka, the DOM. Wikipedia actually has a decent article on the topic: http://en.wikipedia.org/wiki/Doc…
Your design philosophy
I’m not really a huge fan of UIE, but Jared Spool has some decent insights on the anatomy of a design decision. He advocates that it’s important to know what your design philosophy is and embrace it.
Speed and mobile
It’s no surprise that things are changing. Mobile is going to be huge, and performance is going to be a big issue as the web continues to evolve. Paul Irish has a few good insights listed here, so I won’t go into too much detail: https://plus.google.com/11312743…
Nokia’s devs have some interesting tips regarding the best ways to write good JS: http://www.developer.nokia.com/C… as does Google: http://code.google.com/speed/art…