Comment by mustardo

Not even particularly elegant code, for example, opacity is clamped between 0 and 1 in in the very next line a bounds check is made anyway.

  opacity = (opacity < 0) ? 0 : opacity;
  opacity = (opacity > 1) ? 1 : opacity;
  if(opacity >= 0 && opacity <= 1) {
      document.getElementsByTagName("BODY")[0].style.opacity = opacity;
  }