If you go to the link it takes you to a sample of javascript using a confirm dialog box. The code is in a frame to the left and the sample is on the right. After you click the button that says "Display a confirm box" a box will come up with cancel and okay. After you select one of those you should be displayed a message on the page to the right were the first button was. This didn't work in Webkit r.20103 or Safari, but it did work correctly in Camino. I'm running 10.4.8 and built Webkit using the latest Developer Tools.
Confirmed with a local debug build of WebKit r20100 with Safari 2.0.4 (419.3) on Mac OS X 10.4.8 (8N1037). Note that Firefox 2.0.0.2 does complete the document.write(), but it's left in a state where the page load never ends.
Works with no side-effects in Opera 9.10.
*** This bug has been marked as a duplicate of 8961 ***