Q: What is the practical difference between px (pixel) and em (pronounced m)? When is it best to use one over the other?
The short answer is that pixels measure dimensions relative to the screen while ems measure dimensions relative to type size.
The slightly longer answer is that pixels are the natural unit for measuring dimensions on a screen and are often used when precise design is required. Ems are the natural unit for measuring type and used when you want to allow maximum design flexibility.

Many have written about the Three Click Rule. For instance, Jeffrey Zeldman, the influential web designer, wrote about the three click Rule in his popular book, Taking Your Talent to the Web. He writes that the three click Rule is “based on the way people use the Web” and “the rule can help you create [...]

our web site should be easy to read
The most important rule in web design is that your web site should be easy to read. What does this mean? You should choose your text and background colors very carefully. You don’t want to use backgrounds that obscure your text or use colors that are hard to [...]