You cannot style generated content without defining what that content should be. If you don’t really need any content, just an extra “invisible element” to style, you can set it to the empty string (content: '') and just style that.
It’s easy to confirm this yourself: http://jsfiddle.net/mathias/YRm5V/
By the way, the snippet you posted is the micro clearfix hack, which is explained here: http://nicolasgallagher.com/micro-clearfix-hack/
As for your second question, you’ll need an HTML5 shiv (small piece of JavaScript) to make <nav> stylable in some older browsers.