Phrase “xhtml compliant” Means Squat.

The too-often-seen phrase “xhtml compliant” means nothing. Absolutely, 100%, without a doubt, it holds no useful meaning.
That phrase is misleading. It is often the case in English that there are many varieties of something, and we are vague when we assume the other person knows what we mean. We are also vague when we don’t realize that others may not know what we mean. Hence, that other phrase in English, “foolish assumptions.”
Let’s be careful, very careful, to avoid making foolish assumptions. I’ve done it, and I know many of you have, too. You see, there are a few different things that are named XHTML. They’re are not interchangeable. Much like Diesel and Unleaded Gasoline, they’re used for the same basic thing, but you can’t swap them.
XHTML 1.0 Strict (the one I prefer to use)
XHTML 1.0 Transitional (for fools who can’t be bothered to learn an easier set of rules…)
XHTML 1.0 Frameset (for wierdos who really like pain of frames… )
XHTML 1.1 (the one I want to use but can’t yet)

What’s the difference? Well, Transitional just makes you close all your tags,(elements). No more HTML 4 or 3.2 or 2.0 madness of no closing. But otherwise, it’s not so different from HTML 4 but it is really a hybrid of HTML4 and XHTML 1, almost anything goes.
Frameset? Same show with continued support for frames, which are best avoided like the plague. That’s truly the anything goes version.
So Strict is a very different beast. It doesn’t allow frames and the markup must follow new rules (short and simple…) or it will not display properly.
XHTML 1.1 takes things a bit further, dropping all support for old HTML sillyness. But a number of common web browsers still don’t know 1.1 so for now Strict it is.
1.1 will not mean much until CSS3 anyway.

In summary, if something claims to be xhtml compliant, your first question should be “Which xhtml?“