[Constructor(double r, double g, double b, optional double a=1),
   Constructor(RGBAColorInit color)]
  interface RGBAColor {
    attribute double r;
    attribute double g;
    attribute double b;
    attribute double a;
 
    HSLAColorInit asHSL();
    static RGBAColor fromHSLA(double h, double s, double l, optional double a=1);
    static RGBAColor fromHSLA(optional HSLAColorInit color);
 
    HexColorInit asHex();
    static RGBAColor fromHex(octet r, octet g, octet b, optional octet a=255);
    static RGBAColor fromHex(optional HexColorInit color);
 
    DOMString? asName();
 
    DOMString toString(optional DOMString type="rgba");
    static readonly attribute Map serializationTypes;
  };

The toString() method looks up the type in the RGBAColor.serializationTypes Map; if it finds a function, it calls that with the RGBAColor as the sole argument and returns whatever the function returns. It's preloaded with UA-defined functions for “hex3”, “hex6”, “rgb”, “rgba”, “hsl”, and “hsla”.

Authors can define their own color formats by adding a fromFoo() to RGBAColor, a toFoo() to RGBAColor.prototype, and adding an appropriate serialization tag to RGBAColor.serializationTypes.

  dictionary RGBAColorInit {
    double r = 0;
    double g = 0;
    double b = 0;
    double a = 1;
  };
 
  dictionary HSLAColorInit {
    double h = 0;
    double s = 1;
    double l = .5;
    double a = 1;
  };
 
  dictionary HexColorInit {
    octet r = 0;
    octet g = 0;
    octet b = 0;
    octet a = 255;
  };
 
  partial interface CSS {
    RGBAColor parseColor(DOMString color, optional Element el);
  };

parseColor() takes a string containing any CSS color, and returns an RGBAColor for it. If the string isn't parseable as a color, throw a SyntaxError exception.

The optional second argument is used to resolve colors that vary based on the element, such as currentcolor. If such a color is parsed but the second argument is not provided, throw a XXX exception.