I switched the main website from HTML 4.01 Transitional to Strict. I can't remember why I chose to use transitional in the first place. The only deprecated HTML I had was an align attribute in an image tag.
Students in the CS105 class I'm TAing have asked about indenting using straight HTML. This reminded me of a page describing hacks to indent using HTML. (I'd use CSS to do it, but we don't expect them to know CSS well.)