The eternal puzzle of web design validation, with standards being set by the World Wide Web Consortium, is likely to continue for some time. Until such time as all browsers are similarly compliant, we have some careful coding to apply in our web designs, and of course the handy Validation Tool over at the W3C points out any transgressions on the designer’s part!
The guys over at Design Reviver have put together an informative little piece on the most common types of xhtml error. A good starting place to peruse the most common offences, before going deeper into the rabbit hole! This all ties in with the general principles of (World Wide Web Consortium) W3C compliant web design, discussed on the main website briefly.