I get an exception when the html contains the following (from word html):
The method does not handle the "style='MARGIN: 0cm 0cm 0pt'" attribute.
If you use a latest version of Aspose.Words, I think it will simply ignore this attribute without an exception.
At the moment only margin-left, margin-right, margin-top and margin-bottom are processed, a compoind margin property like in your example is ignored.
Let us know if it is still a problem.