Was testing a form I'd designed using our CMS editor, got it to display in every browser, except IE9. IE9 would not style my form. It would style the rest of the site, but not the form.
Turns out, with my form wrapped in two sets of P tags (split due to the CMS limitation) IE was the only browser to complain that it was invalid and make me fix it
IE9 is really unforgiving of invalid code