I love the concept of "Pixels", but I haven't had a chance to check it out until today. Unfortunately it doesn't seem to be working anymore, unless I'm missing something. When I go there, I'm able to zoom the first level just fine, but then it fades away to pure white and there are no additional levels to be seen. I've tried it on Mac Firefox and Mac Chrome, and I've zoomed in at a number of locations within the image, and I've tried letting it sit for a while, all to no avail. When I look in the developer console, I see a couple of files are failing to load:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://c.xkcd.com/turtle/turtles. This can be fixed by moving the resource to the same domain or enabling CORS.
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://c.xkcd.com/cdndata. This can be fixed by moving the resource to the same domain or enabling CORS.

They both timeout after about 90 seconds.