According to Google bot engineers, who posted a "code survey" last year analyzing how just over a billion web pages use various tags, Googlebot is "validation ignorant". The bot only cares if the code can be parsed, and the parser is much more basic than W3C. As long as the tag is opened and closed properly, the bot will process it.

The wording in the webmaster guidelines is "valid" not "validated". The standard that Google (and other SE) reps usually point to is the Lynx browser - if Lynx displays it, the bots can probably read it.

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