It appears to be the case that if you call JQuery's animate() passing any set of options, in Webkit browsers, where the starting value and ending value for e.g. top or left are the same, the browser will elect to animate them anyway, setting them to zero before starting the animation. This was throwing out all my custom animations in Chrome and Epiphany. So I've rewritten the code so that where a value is not going to be altered, it's never actually passed in to the animate() call. The presentation is now functioning correctly on FF, Webkit and Opera engines.