I happened to be checking the validation of moodle.org at:

http://validator.w3.org/check?uri=http%3A%2F%2Fmoodle.org%2F&charset=%28detect+automatically%29&doctype=Inline&group=0&user-agent=W3C_Validator%2F1.1

and noticed the following:

Validation Output: 1 Error

1. Error Line 38, Column 4398: document type does not allow element "input" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag

...ine"><input type="submit" value="Go" /></noscript></div></form></div> ...

I'm not sure why the input type is declared inside the noscript. I looked at /lib/outputrenderers.php ~1175

Petr seems to have introduced this at:
http://cvs.moodle.org/moodle/lib/outputrenderers.php?view=diff&r1=1.156&r2=1.157
in relation to MDL-20204

I don't see the need for the noscript but I suspect that the validation is complaining because it is not accepting <input> inside of the <noscript> tag. On my server I simply replaced it with:

$output .= html_writer::empty_tag('input', array('type'=>'submit', 'value'=>get_string('go')));
// $go = html_writer::empty_tag('input', array('type'=>'submit', 'value'=>get_string('go')));
// $output .= html_writer::tag('noscript', $go, array('style'=>'inline'));

and then the page validated fine. Peace - Anthony