That said, here are some of the top 10 xhtml errors by philwebsupport:
1. The use of a raw amperstand in a link query string. The w3c validator reports this as “cannot generate system identifier for general entity” because you’ve tried to create a new entity &xxxxxxx and not an encoded & amp ; in the string. Replace all & with & in urls.
2. The forgotten alt tag. The w3c validator reports this as “required attribute “alt” not specified,” which means that for every img tag you have, you must have an attribute alt=”something”. So, what you need to do is change 
3. Missing end tags. The w3c validator reports this as “end tag for “img” omitted, but OMITTAG NO was specified” for your particular tag–what it means is that you used a singleton tag, that is, a tag that stands by itself and doesn’t have an inherent end tag, so you must use the xml style / delimiter to signify that the tag ends itself. So, instead of
4. Incorrect nesting of lists. Please do not place lists inside a paragraph tag. The w3c reports this error as “document type does not allow element “ul” here; missing one of “object”, “applet”, “map”, “iframe”, “button”, “ins”, “del” start-tag.”
5. Incorrect nesting of tags. Think of tags as a stack–as you add new tags to your text, you close the most immediate one first, or you’ll get errors like this: “end tag for “strong” omitted, but OMITTAG NO was specified” and “end tag for element “strong” which is not open.” Instead, change to
6. Oh, the horrors of flash. Did you know it’s really hard to embed flash properly? Luckily, the problem has been solved by people: www.alistapart.com/articles/flashsatay/ who basically took the Macromedia output and stripped it down. Sad, though, that they didn’t build up from the spec…
7. Where’s the doctype? Again, ALA to the rescue with an informative article on document typing: www.alistapart.com/articles/doctype/. If your site doesn’t have a doctype, it’s not a well-formed html document!
8. Javascript events are lowercase. How many times have you followed standard coding conventions and written your onclick handler as an onClick handler? Probably too many to tell. Just make it lowercase, and that “there is no attribute “onClick”” will go away!
9. Using propietary CSS extensions. Even if you’re tempted to use the word-wrap property on a blockquote or a right float, don’t. The microsoft or Mozilla-only CSS extensions aren’t good down the road when you want to upgrade your site technology.
10. Some things need a type, javascript for example. If you forget the type=”text/javascript” from your script declaration, or the type=”text/css” from your stylesheet, it won’t validate, for obvious reasons.
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