font-variant: normal | small-caps | prince-no-kerning | prince-opentype([ feature ]+)
<span style="font-family: SpecialFont; font-variant: prince-opentype(aalt(2))">&</span>
font-variant: prince-opentype(onum, ccmp, liga)
The prince-opentype() function takes a comma-separated list of OpenType features. Microsoft has a list of the OpenType feature names here.
Some OpenType features like aalt (access all alternates) can be used to select one of many optional glyphs, eg. a font might provide three different ampersand glyphs and allow the author to choose which one. An optional integer parameter is how you do this, however it requires intimate details of exactly which font is being used and what glyphs it provides.
The feature can be written either in function form (feature(N)) or in concatenated form (featureN).