<img style="width:400px" src="http://img.hb.aicdn.com/787bf87d05ad774522dd92151b3051b463229a11109598-9QXV9C_fw658">
<script>
var imgW = document.getElementsByTagName('img')[0].style.width;
  alert(imgW); //返回值为400px，否则返回空；
</script>

<style>
img{ padding:20px;border:1px solid red;}
</style>
<img style="width:400px" src="http://img.hb.aicdn.com/787bf87d05ad774522dd92151b3051b463229a11109598-9QXV9C_fw658">
<script>
var img = document.getElementsByTagName('img')[0], 
      imgOffsetWidth = img.offsetWidth, //442px
      imgClientWidth = img.clientWidth; //440px;
</script>

<img style="width: 400px;" src="http://img.hb.aicdn.com/787bf87d05ad774522dd92151b3051b463229a11109598-9QXV9C_fw658">

  <script>
    function getStyle(el, name) {
      if(window.getComputedStyle) {
        return window.getComputedStyle(el, null)[name];
      }else{
        return el.currentStyle[name];
      }
    }
    var div = document.getElementsByTagName('img')[0];
    alert(getStyle(div, 'width'));
  </script>

<img style="width: 400px;" src="http://img.hb.aicdn.com/787bf87d05ad774522dd92151b3051b463229a11109598-9QXV9C_fw658">

  <script>
    document.addEventListener('DOMContentLoaded',function(){
      function getImgNaturalStyle(img,callback) {
        var nWidth, nHeight
        if (img.naturalWidth) { // 现代浏览器
          nWidth = img.naturalWidth
          nHeight = img.naturalHeight
        } else { // IE6/7/8
          var imgae = new Image();
          image.src = img.src;
          image.onload = function(){
            callback(image.width, image.height)
          }
        }
        return [nWidth, nHeight]
      }
      var img = document.getElementsByTagName('img')[0],
          imgNatural = getImgNaturalStyle(img);
      alert(imgNatural);
    });
  </script>

document.addEventListener("DOMContentLoaded",function(){

    //原因就是当时我们的代码是在这样的环境下写的，这个时候，只是加载了img的标签，即只有DOM结构，图片还没有完全加载进来，所以获取到的值都是0，但如果在window.onloaded的环境下写，就能得到其所示高宽了

});