Related to:

http://bugs.jquery.com/ticket/5001

In jQuery (1.5 and below), the .animate() step: method will add a unit of "px" even when animating the z-index property. This doesn't trouble modern browsers, but IE8 and below don't understand a value of "z-index: 0px" and throws an error, failing entirely.

The related ticket linked above is closed as WontFix, but I think this particular use case is quite legitimate and should be fixed. Animating on z-index is a great way to do custom run loops (which I just wrote a plugin for, hence how this got discovered) in a way that doesn't cause reflow and doesn't require constant unit processing.

I fixed it in my own version by doing this crude fix (lines 7768-7775):

		_default: function( fx ) {
			if ( fx.elem.style && fx.elem.style[ fx.prop ] != null ) {
				fx.unit = ( fx.prop == "zIndex" ) ? "" : fx.unit; /* Bug fix : Faruk Ates */
				fx.elem.style[ fx.prop ] = (fx.prop === "width" || fx.prop === "height" ? Math.max(0, fx.now) : fx.now) + fx.unit;
			} else {
				fx.elem[ fx.prop ] = fx.now;
			}
		}