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Until HTML5, IDs can only start with a letter, but if I have an ID starting with an underscore in my document, it still validates as XHTML 1.0 Strict.
Created attachment 969 [details] sample document
After posting this bug, I've found out that IDs starting with underscores are valid in XHTML 1.0. http://lists.w3.org/Archives/Public/www-validator/2010May/0028.html