Congratulations to the Opera team for being the first to pass the Acid3 test. Acid3 tests much more than Acid2, which you may have heard about recently. It tests various areas of CSS, the DOM and I forgot the rest. In other browsers, Safari is at something like 96/100 and FF is in the upper 80s to low 90s. IE7 is a horrible 12. IE8 beta1 is a dismal 17.
After Opera passed the test (the nightly build for the next release), Apple noticed a small bug in the test. The test was changed and makes Opera fall back to 99/100. So if you change the rules then you can't expect things to stay the same. In any case, I would bet the new rule will be targeted and fixed soon. More importantly, they passed what they were given.