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DTD-based validators are technically not able to test for these types of attribute value problems.
Pages may not display as intended in all browsers, even in the absence of validation errors and successful display in other browsers.
Mark-up validators cannot see the "big picture" on a web page, but they excel at picking up missed closing tags and other technicalities.
DTD-based validators are also limited in their ability to check attribute values according to many specification documents.
If you see this logo on a site, what does it mean to you? We just went through the exercise of validating Stack Overflow's HTML. Anyway, we validated as the much saner HTML 4.01 strict, and even then I'm not sure it was worth the time we spent.