As a web developer there are quite a few things you need to know and preferrably remember to deal with the biggest nightmare since the boogieman: Browser Compatilibity. At the server-side one has full control over the language and the environment one prefers to comply to, but at the client - it should just work no matter what browser the user's on. Quirksmode has put together a nice crosstable of the most hazerdous CSS selectors and declarations and specified to which degree they work in the various browsers.
I basically just discovered the page, and have already found much use for it. Even got surprised, in a positive way. Quite some time ago I had some problems with in particilar the attribute selectors, and decided that they'd be a bad idea to use. Turns out Internet Explorer 7 just supports them, which means I can use them without being afraid.
Internet Explorer 6 support is being dropped on a web-global scale anyway.
Essential reference for any web developer who takes themselves serious!te