Why doesn't this a:visited css style work?

Actually, this has nothing to do with case sensitivity. This is a security feature. The functionality of :visited pseudoclass has been restricted in many modern browsers (Fx4, IE9, Chrome) to prevent CSS exploit: read about it here.

Nowadays, getComputedStyle() in these browsers usually returns values for visited links as if they weren't visited. However, I can simply imagine circumventing of that: using font-weight for visited links, the element's width changes so browsers that would allow changing font-weight for :visited links wouldn't actually fix the security hole.

Thus, there's no workaround for this issue.

As of 2020 one useful attribute that still works with :visited is background-color. So try:

:visited {background-color:red !important;}

:visited also works on non a elements.

(Note: If its working, remove !important. Bad practice. Using it here just to be quick.)