The Selectors API querySelector and querySelectorAll methods won't match uppercase class names in quirks mode.
e.g. Try running this script in quirks mode.
var p = document.querySelector(".FOO");
document.write(p ? "PASS" : "FAIL");
The same script works correctly in standards mode.
note, this also affects getElementsByClassName in quirks mode
I can't reproduce this (I pasted the provided testcase into a document and loaded it in safari 4 beta, and it said PASS, same with r40738).