OK, let me not keep you in suspense for too long: this looks like some JavaScript issue to me, probably independent of Mac model or browser
*. FWIW, I see exactly the same thing as you do, with the CPU maxing out as soon as the countdown reaches zero. When I now disable JavaScript in Safari's prefs, the animation immediately resumes its normal sequence and the CPU load drops below max. Running OS X 10.5. 8 and Safari 5.0.6 on a PPC iMac iSight (temporarily no Intel Macs at hand to test).
*) I'd bet the WU tech is not testing with a Mac. The affected WU functionality was probably caused by changes to the various animation pages around the time you started to notice this. Oh, and if you test the JavaScript effect, make sure to turn it back ON if you want to post the result here...
