Click to see this page at a size that demonstrates the bug

The bug is that a border drawn in the double style sometimes gets compressed into a single line. Clicking on the link below will pop up a window with this same page, but smaller. There you will see that the 4th border is compressed into a single line instead of being shown as a double line as it should.

This bug occurs on WinXP using 1.5b1. This is not a problem in Firefox 1.0.7, so this should be marked a REGRESSION.