Hello,
I had problems with the "frac" feature of opentype in Prince.
This is my CSS:

span.fraction {
  font-variant: prince-opentype(frac);
}

And my XHTML:

<p>This is a fraction: <span class="fraction">1/6</span></p>

This did not work, because you must use the right character for the fraction bar: not the simple slash ("/", unicode "Solidus", 0x002F), but the unicode "fraction slash" (0x2044).

This is the right XHTML (that looks the same, except for the "fraction slash"):

<p>This is a fraction: <span class="fraction">1⁄6</span></p>

Great, glad to hear it is working now.