Actual implementations are currently CSS extensions as the functionality is not yet in a published spec.
Mozilla's CSS extension uses the syntax:
Microsoft's CSS extension however uses the same syntax as the current W3C draft, expect with the standard browser specific prefix used before a feature makes it into the official spec.
-ms-font-feature-settings: "kern" 1;
Whether this syntax is ideal or not is still, imho, contestable. Either way it isn't Mozilla's parameter-esque style (which I agree stands very much in opposition to the usual CSS syntax style), and as long as the syntax in the draft reaches finalization, said parameter-esque syntax should go away for good.