n>
      <span style="top: 0px; width: 200px"></span>

      <!-- cm -->
      <span style="top: 30px"><math><mspace width="5.08cm"/></math></span>
      <span style="top: 20px; width: 192px"></span>

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

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