The danger of validating your XHTML
The danger of validating XHTML is that the validation is almost certainly not doing what you believe it's doing.
The problem is that all the common online validators ignore the HTTP
Content-Type of what your web server returns when validating your page,
and use only the DOCTYPE. This is completely wrong in the case of XHTML,
because browsers only treat pages as XHTML if they are served as
application/xhtml+xml. All the DOCTYPE does is let the browser decide
what sort of XHTML it has, since there are now several flavours.
(No less authority than the W3C says that browsers should behave this way; see this mailing list message, or this Safari blog entry, or even the W3C's XHTML media types note.)
So if you serve your beautifully validated XHTML as
browsers will actually see it as is good old HTML tag soup. And
validators won't tell you this; they will happily tell you that your
text/html page is valid XHTML, when an honest answer is that it is
(In some future world it may be valid HTML5.)
If you are merely using XHTML validation as a good housekeeping seal of approval, you should write to HTML 4.01 Strict instead; it is just as strong, and browsers will actually interpret your pages the way you think they are, saving you various headaches.
If you are using XHTML validation to prepare for serving your pages
application/xhtml+xml in the future, you are probably fooling
yourself, because your pages may or may not actually work as real XHTML
(see some of the links here for a full explanation).
If you are trying to serve real XHTML to selected browsers and your XHTML document as HTML to Internet Explorer, and you are not using either MathML or SVG, you are a masochist.