Doesn't work for non-touchscreen browsers

[OllyHodgson]

This looks great :) Alas it doesn't work on non-touch (e.g. desktop) browsers that happen to be narrow. I'm guessing you'll want to check for the availability of touch events in your code, e.g.

if ('ontouchstart' in document.documentElement === true) {
    /* do stuff */ 
}

[tkenny]

It only works on page load so resizing a desktop browser won't work. You have to refresh when it's narrow enough. I tried it so that it would change on resize but it caused some issues.

[OllyHodgson]

Ah, I see the problem now. It's fine in Chrome, but in both IE9 and Firefox the z-index puts nav#mobile behind div#site.

(by the way, the lack of a doctype drops IE9 into quirks mode)