It appears the blur mechanism for the box-shadow in webkit (tested in Google Chrome and Safari) produces particularly poor results. As a designer it gets my attention quickly. Detail really is important and inconsistent rendering across platforms is really bad. I've done screen grabs comparing to Gecko/Firefox and a reference rendering using Gaussian blur. Firefox produces fairly good results. It may also be poor for text-shadow but right now I don't have time to document it. I suspect they both use the same blurring algorithm. I've attached the screen grab in addition to providing a URL demonstrating the problem and showing a live example as well as the screen grab. This is just for convenience.
Created attachment 53048 [details] Screengrabs of box-shadow rendering in webkit, firefox and a reference one I made myself in a graphics program using guassian blur.
Latest Webkit Nightlies show a vast improvement in rendering quality. However it is still different to the firefox and reference rendering. Since the draft spec has been pulled at the moment I don't think this really counts for a bug anymore. Apologies for any inconvenience, I've been working off old notes when filing this.