Validation.

I went through css hell and back again this week.

And reading through online posts for help, I discovered that generally there are two schools of thought on old-school html vs. new school xhtml: why bother to convert to css/who-the-f***-cares vs. standards rule/I'm-more-compliant-than-you-are!

And I found out I am in the latter camp (somewhat begrudgingly).

Working with tables, I was about to pull my hair out, and working with css is the same way, only with a small, dim light at the end of the tunnel.

As a big proponent of interoperability (of all kinds), I understand the need for standards.

Additionally, it's somewhat akin to learning art techniques... You don't necessarily have to know the rules to break them, but knowing the rules allows you to break them with intention, panache, and an understanding of the consequences that follow.

That said, the css/xhtml learning curve is steep...