I do not want to sound like I’m talking out of both sides of my mouth, but please keep in mind that the validator is a tool, arguably the most valuable in your toolkit, but still just a tool. One can write perfectly valid code that doesn’t render a thing on the screen or renders a messy layout. A solid understanding of the fundamentals of HTML and CSS is the best tool of all and opens the door to a greater understanding of the intricacies of how they work together.and interact with one another. There are often several ways of doing the same things using CSS, but the best choice depends on the context in which the behavior is needed.
Want to read an interesting article? Paul OBrien just started a new thread about positioning objects on a page. Fascinating reading.