The 'padding' between the enumerator and denominator of a mfrac bevelled is quite large, making the result look quite bad. See the screenshot. The fraction should be drawn much smaller, preferably something like:
http://seriot.ch/images/latex/fractions.png (just a random image I found)
Here's code to reproduce (I'm using Prince 9 on Mac OS X 10.8):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN"
"http://www.w3.org/Math/DTD/mathml2/xhtml-math11-f.dtd">
<HTML xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<BODY>
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow><mi>12345</mi><mfrac bevelled="true"><mn>1</mn><mn>4</mn></mfrac><mi>12345</mi></mrow>
</math>
</BODY>
</HTML>
http://seriot.ch/images/latex/fractions.png (just a random image I found)
Here's code to reproduce (I'm using Prince 9 on Mac OS X 10.8):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN"
"http://www.w3.org/Math/DTD/mathml2/xhtml-math11-f.dtd">
<HTML xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<BODY>
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow><mi>12345</mi><mfrac bevelled="true"><mn>1</mn><mn>4</mn></mfrac><mi>12345</mi></mrow>
</math>
</BODY>
</HTML>