@Override
 public void add(int index, Layer layer) {
   Asserts.checkArgument(layer instanceof HtmlLayerGL);
   impl.add(this, index, (HtmlLayerGL) layer);
 }
 @Override
 public void remove(Layer layer) {
   Asserts.checkArgument(layer instanceof HtmlLayerGL);
   impl.remove(this, (HtmlLayerGL) layer);
 }
 @Override
 public float width() {
   Asserts.checkNotNull(surface, "Surface must not be null");
   return surface.width();
 }