<body><p>  a<em>b<span>c</em>d</span>e  </p></body>

  Mozilla in Quirks mode marks up as: 
<body><p>  a<em>b<span>c</span></em>de  </p></body>

  Opera in Quirks mode marks up as:
<body><p>  a<em>b<span>c</span></em><span>d</span>e  </p></body>

  IE in Quirks mode marks up as:
<body><p>  a<em>b<span>c</span></em><span>d</span>e  </p></body>

Interacting with IE and Opera indicates that the DOM is different in the two cases