It’s an incredibly simple rule of thumb and one that I catch myself out with time and time again, spending twenty minutes pulling my hair out with FireBug open trying to work out why the bl**dy thing isn’t behaving as I expected it to (I hope it’s fair to say everyone has days like that though!)

Don't Start a CSS Selector with a Numeral

Don’t start CSS class or ID names with a numeric character!

I’ve never been able to find any ‘official’ documentation about this but the principals are essentially the same as any other parsed programming language: it’s very pot luck whether selector names will work in your CSS if they start with a numeric character. I don’t know if this is just a limitation on the sort of environment I tend to develop in (FireFox with FireBug) but the resolution is very simple: always start your ID and Class names with an alphabetic character and not a number. Which is a little annoying since the website I’ve been working on is called 2degrees…!