I have a rotate animation, and I got the numbers by opening the exact same image in Gimp and using the rotate tool there to position the center of rotation. In Gimp I can spin the image around this center of rotation and it works fine, but when I copy the exact values from Gimp into the css animation the animation uses a different center of rotation. In Gimp it is 50.34% 64.17%, and what I had to work out by fiddling to get close to the same center in the css was 50.70% 63.5%.
What's with this? I can use this center, but I am still curious why Gimp's couldn't work. I noticed that removing the width: and height: styling from the image changed the rotation center, but didn't make it the same as in Gimp.
If you don't mind merely satisfying my curiosity!