JQuery class selectors like $(.someClass) are case sensitive? (1)
Moral of the story: match case of everything in your HTML (element names, CSS classes, etc.) because you never know when a change to a browser or standard or library might invalidate your assumption about case-insensitivity.
Given this HTML:
<div class="OpenIDSelector">some text</div>
Why does this JQuery selector match it on some browsers and some pages, but not on others?
NOTE: I ran into this problem and solved it myself, but it was annoying and I didn't find it on StackOverflow already, so I'm posting it as a Q&A pair so someone else won't waste an hour like I did.