Why does adding float:left to my css make my link unclickable?

The fix is pretty simple and cross browser too, add this (to your unclickable link):

#QuestionEditLink, #QuestionHistoryLink {
    position: relative;
    z-index: 10;
}

The fix here is the z-index, but it won’t work if position isn’t relative/absolute/fixed.

For more info on z-index see https://developer.mozilla.org/en-US/docs/Web/CSS/z-index

Have used this all the time, is simple, works in all browsers (IE6+, FF, Chrome, Safari, Opera).

Since others already said about your CSS issues there, I won’t add to it. Just providing a solution, by the way, tested it with your provided JSBin, worked as always!