模态框遮罩层，兼容IE

css:
 .overlay {
      position: fixed; height: 100%; width: 100%;
      filter: progid:DXImageTransform.Microsoft.gradient(startcolorstr=#7F000000,endcolorstr=#7F000000);
      background-color: rgba(0,0,0,.5);
      overflow: auto;
  }
  :root .overlay {
      /* IE9+ 支持:root,支持rgba，故把filter设为none */
      filter: none;
  }

html: 
<div class="overlay"></div>