/span>
      <span style="top: 30px; width: 192px"></span>

      <!-- em -->
      <span style="top: 40px"><math><mspace width="20em"/></math></span>
      <span style="top: 50px; width: 200px"></span>

      <!-- ex -->
      <span style="top: 60px"><math><mspace width="30ex"/></math></span>
      <span style="top: 70px; width: 30ex"></span>

      <!-- in -->
      <span style="top: 80px"><math><mspace width="2in"/></math></span>
      <span style="top: 90px; width: 192px"></span>

      <!-- mm -->
    