The combined border property is null, regardless of what it is actually set to. The expanded properties are correct.
Created attachment 6991 [details] Test Case
Created attachment 6992 [details] screenshot
This was improved in r15703. We still need to make the border shorthand work.
This is the same issue as bug 13658, which talks more generally about shorthand property values not working correctly on computed styles. *** This bug has been marked as a duplicate of 13658 ***