grandchild4 {width: 20em; background: green; border-right: solid 1em red;
    margin-right: -1em}
  </style>
 </head>

 <body>

   <p>There should be no red.</p>

   <div id="container">
     <div id="child1">
       <div id="grandchild1">
	 Block 1
       </div>
     </div>

     <div id="child2">
       <div id="grandchild2">
	 Block 2
       </div>
     </div>

     <div id="child3">
       <img id="grandchild3" src="support/1x1-green.png" alt="Image download support must be enabled" />
     </div>

    