When you quantify how much leverage has been given to vendor politics over common sense, XHTML is the epitome of where the marriage of validity and well-formedness did much more harm than good. To this day, I do not understand why even well-informed people believe there is no value in XML without validation. For me, the sad story that is (or was?) XHTML says it all, even much more loudly than evidence of the same dowry being paid by the next generation of XML standards and WS-*
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