If your CSS stylesheet seems broken

Forgetting a semi-colon can do a lot of damage too.

Also make sure you have closed all your comments. Amazingly some people don’t know they have to do this.
<!-- comment goes in here --> for embedded css
/* comment goes in here */ for external stylesheet

People – this is why there’s a CSS validator. Use it. It will save you a lot of headaches.

Alas, ’tis the people who most need to use it who don’t (won’t, or don’t even know it exists).