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).
Created attachment 461503 [details]
Quirk Mode - Test Case
I changed the test case from Comment 0 in JSfiddle below:
Link - https://jsfiddle.net/6r9odj5x/show
All browsers (Chrome Canary 106, Firefox Nightly 105 and Safari 15.6 on macOS 12.5) are saying "PASS" for the test case, I am going to mark this as "RESOLVED CONFIGURATION CHANGED"
Even with above "Quirk Mode" test case, all browsers are showing "PASS". Thanks!