It's just a suggestion, may be it can lead to problems.
As all IP codes are valid - http://validator.w3.org/
the only one I found which is not is Target = _blank when you define a link in Rich/Text.
If a link is defined to open in a new page, the XHTML validation is invalid.there is no attribute "target"
…t" href="http://www.apple.com" target="_blank">external</a> test. Valid XHML S…
You have used the attribute named above in your document, but the document type you are using does not support that attribute for this element. This error is often caused by incorrect use of the "Strict" document type with a document that uses frames (e.g. you must use the "Transitional" document type to get the "target" attribute), or by using vendor proprietary extensions such as "marginheight" (this is usually fixed by using CSS to achieve the desired effect instead).
How to fix: check the spelling and case of the element and attribute, (Remember XHTML is all lower-case) and/or check that they are both allowed in the chosen document type, and/or use CSS instead of this attribute. If you received this error when using the <embed> element to incorporate flash media in a Web page, see the FAQ item on valid flash.
This error may also result if the element itself is not supported in the document type you are using, as an undefined element will have no supported attributes; in this case, see the element-undefined error message for further information.
Here are few solutions:http://snipplr.com/view/2245/
Another one (not recommended by author of previous one)http://snipplr.com/view/282/
some comments provides enhancements and links below other solutions...