Why em instead of px?

Question

I heard you should define sizes and distances in your stylesheet with em instead of in pixels. So the question is why should I use em instead of px when defining styles in c

Answer 1

You probably want to use em for font sizes until IE6 is gone (from your site). Px will be alright when page zooming (as opposed to text zooming) becomes the standard behaviour.

Traingamer already provided the neccessary links.