Try these. The text in the second, third, and fourth will be invisible in Safari but will be visible in other browsers.
alert("abcdefaghdfdsgfdsfdsfsdfsefsdfdsfdsfdsfdsddfsfdsrf seferefre ew");
alert("12345678901234567890123456789012345678901 0564056 0506");
This is a Safari bug, not a WebKit one. Safari bug reports should be filed at http://bugreport.apple.com/ This bug, however, has already been filed, so you won't need to file a new one.
If I'm mistaken, this is Windows-only as well.