Acid3 Test score 94/100, 4th box rendered gray, PowerPC G5, 10.5.2, r33023
Correction: The 5th box is rendered gray.
Please check Help -> Installed Plug-ins to see whether you have the Adobe SVG plug-in installed. If you do, remove it from /Library/Internet Plug-ins/, restart Safari and try again. This is by far the most common reason that people have reported seeing lower-than-usual scores on PowerPC machines.
I did not have that specific plug-in, but I did find under Help -> Installed Plug-ins NPSVG3Carbon, which was described as an "SVG Document" created 10/26/01. I removed NPSVG3Carbon from /Library/Internet Plug-ins. Now Acid3 Test score 100/100. Thanks.