Suppose you're sold on the idea of XHTML instead of HTML. Then, the next thing you have to decide is whether to go for XHTML 1.0 Strict or XHTML 1.0 Transitional.
The names speak for themselves. Strict is strict/rigid, Transitional is transitional/forgiving. The benefits of Strict over Transitional are similar to those of XHTML over HTML in principle. The question is, is Transitional not good enough? Do you have to go so far as to use Strict?
Jeffery Zeldman writes in his book, Designing With Web Standards
"In XHTML 1.0 and 1.1 Strict, presentational hacks are actually forbidden: Use them and your page will no longer pass muster when you run it through the W3C's Markup Validation Service, familiarly known as "the Validator"."
He also gets practical with it in this paragraph:
"Many standards geeks prefer XHTML 1.1 Strict, and there's nothing wrong with it, but it is less compatible with the past and it is properly served with a MIME type that causes some popular current browsers to misbehave. Additionally, converting old-school web pages to XHTML 1.1 Strict takes more work and requires more rethinking than converting to XHTML 1.0 Transitional. For many readers of this book, XHTML 1.0 Transitional is likely to be the best choice for the next few years."
Ok. I think I get it now. Use Transitional when converting an old HTML site over to XHTML. Use Strict when you're starting something new.
Another such variable to consider is the CMS (Content Management System). My system, Zope generates image tags that are invalid for Strict because they have, by default, a
border="0" attribute by default.