Validity and well-formedness
I’ve just published a new web development article called Validity and Well-Formedness, which explains the distinctions between valid and well-formed XHTML.
If the W3C HTML Validator says your XHTML page is valid, that means it’s also well-formed, right? Wrong! This article has several examples of XHTML documents which are perfectly valid but are malformed and won’t even load in an XML parser.