It should be possible to say:

  void drawImage(in HTMLImageElement image, in float dx, in float dy, in optional float dw, in float dh);

...so that it means the same as:

  void drawImage(in HTMLImageElement image, in float dx, in float dy);
  void drawImage(in HTMLImageElement image, in float dx, in float dy, in float dw, in float dh);

Basically, have "optional" means that if the argument is omitted, all subsequent ones must be too, but that if it is included, any subsequent arguments up to the next "optional" must be included.

Currently, "optional" can't be omitted on arguments after the first with "optional", so the above isn't possible.