If a procedure to deal with certain markup is defined, that markup is not errorneous anymore. This is a highy theoretical reason, but there is a practical one. You could come up with a better handling than the recomended one and in that case your handling would be considered errorneous. I think there can't be unbiased statistical evidence.
Author writes some HTML and uses a browser to preview it.
If he is not satisfied, he does some mumbo-jumbo with the tags and looks again. If it looks good now, that's the final document. Except that it's not guessing any more.