test for : msline mslinethickness="thin" (with mathsize="18pt")
<math xmlns="http://www.w3.org/1998/Math/MathML"> <mstyle mathsize="18pt"> <mstack> <mn>424</mn> <msrow> <mo>+</mo> <none/> <mn>33</mn> </msrow> <msline mslinethickness="thin"/> </mstack> </mstyle> </math>
Author: Yecheng Gu