Some time ago I wrote a blog post Firefox 4.0 Features for Font Fanatics. Since that time the spec has evolved and support has arrived in Chrome and IE10 so I thought it was worth an update.
Here's the example CSS rule from the original post:

.example {
    font-feature-settings: "dlig=1,tnum=1,ss01=1";
}

This is what the syntax now looks like:

.example {
    font-feature-settings: "dlig" 1, "tnum" 1, "ss01" 1;
}

As you can see the change is subtle. For practical support you'll need to use vendor prefixes, of course:

.example {
    -moz-font-feature-settings: "dlig" 1, "tnum" 1, "ss01" 1;
    -webkit-font-feature-settings: "dlig" 1, "tnum" 1, "ss01" 1;
    -ms-font-feature-settings: "dlig" 1, "tnum" 1, "ss01" 1;
}

I've updated my examples to use the new syntax:

• Ligatures
• Numbers
• Kerning