UserJS to maximize image by double-click on Opera

MaximizeImg.js

// ==UserScript==
// @description maximize image by double-click instead of auto resizing.
// @author Chihiro Ito
// ==/UserScript==

if(document.querySelector("head>link[rel='stylesheet'][href='opera:style/image.css']")) {
  window.donotrun=true;
  document.body.style='margin:0; padding:0;';
  var img = document.getElementsByTagName('img')[0];
  if(img) {
    var prevTime = 0;
    img.addEventListener('mousedown', function(e) {
      var time = (new Date()).getTime();
      if(time - prevTime > 500) {
        prevTime = time;
        return;
      }
      prevTime = 0;
      if((img.style.width  && img.style.width != 'auto') ||
         (img.style.height && img.style.height != 'auto')) {
        img.style.width = img.style.height = 'auto';
      } else {
        var clientWidth = 0,clientHeight = 0;
        if(window.innerWidth){
          clientWidth  = window.innerWidth;
          clientHeight = window.innerHeight;
        } else if(document.documentElement && document.documentElement.clientWidth){
          clientWidth  = document.documentElement.clientWidth;
          clientHeight = document.documentElement.clientHeight;
        } else if(document.body){
          clientWidth  = document.body.clientWidth;
          clientHeight = document.body.clientHeight;
        }
        var imageAspect  = img.width / img.height;
        var clientAspect = clientWidth / clientHeight;
        if(imageAspect <= clientAspect) {
          img.style.width  = (clientHeight * imageAspect) + 'px';
          img.style.height = clientHeight + 'px';
        } else {
          img.style.width  = clientWidth + 'px';
          img.style.height = (clientWidth / imageAspect) + 'px';
        }
      }
    }, false);
  }
}