In my standard stylesheet.css I use when redoing a Project Gutenberg book for the body tag I have
Code:
body {
border: 0;
font-feature-settings: "kern", "liga", "clig", "onum", "pnum";
font-kerning: normal;
font-style: normal;
font-variant: common-ligatures oldstyle-nums proportional-nums;
hyphens: auto;
margin: 0;
padding: 0;
text-rendering: optimizeLegibility;
width: auto;
}
I don't remember which ones are needed for old style figures but probably the font-variant: oldstyle-nums and the font-feature-settings: onum are what's needed.