Over the past couple weeks, I've had very sporadic reports that Quizlet was completely failing on some computers. This afternoon I spent an hour on the phone working with a very helpful Quizlet user who was able to help me find the problem.
As I understand it, Quizlet was causing some older computers to freeze as soon as they tried to load the page. They were using Internet Explorer, but I'm not sure what version (this user was using it on Windows ME). This was happening since I introduced Space Race and along with it the shiny new expandable buttons. It was not happening on IE 7, and not the majority of IE 6. But it was causing problems for enough computers to be very annoying!
I started by disabling some of the compression features on Quizlet, but they still crashed the computer. Next I transfered the HTML onto a separate test server and had him reload the page and tell me whether the computer crashed or not. I narrowed it down to an external file, then to the main css file (with 1,800+ lines), then to the bottom half of it, then down to a single rule block, then finally to a single CSS rule:
white-space: nowrap. Why that single rule would cause a problem in Internet Explorer is anyone's guess, but it does cause it to crash, and removing it made the site work fine again. This problem has been documented, but it's so obscure that most developers probably don't know about it. This is the kind of bug that's impossible to predict.
So long story short: Welcome back Internet Explorer users! I'm very sorry you couldn't use the site for the last couple weeks.
Last quick message: this is just one more reason to switch to Firefox, which very rarely has problems like this and should always work with Quizlet (and every other site). Its completely capable of replacing Internet Explorer, and I personally recommend it all the time. You'll like it! :)