There is a function in comment-template.php that assigns "_wp_unfiltered_html_comment" ID and NAME to a hidden <input>. This markup causes validation warning.
W3C Specification ( http://www.w3.org/TR/html4/types.html#h-6.2 ) says:
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
It would be very nice to fix this.