I've been harping on people lately about using deprecated or invalid HTML, and I was just wondering how important you guys feel it is to use up-to-date valid HTML?

I know things like <font> and <center> will probably still be supported by most browsers for years to come, so is it really that important?

I've made it a point to try to keep my documents up to HTML 4.01 standards, and now I'm working on updating everything to XHTML 1.0.

What are your thoughts?