Comment 2 for bug 379409

It looks like cssutils, when encountering a rule that contains something like "top_height: 0px", automatically replaces "0px" by "0", assuming 0 is absolute and therefore the unit doesn't matter.

Possible solutions:
1) See if there is a way to tell cssutils not to discard the unit
2) When applying the style properties to the PiecePanel widget, if a value of 0 is given, substitute it with ValueWithUnit(0, "px").