<!DOCTYPE html>
<html style="height: 100%">
  <body style="height: 100%">
    <fieldset style="position: relative; display: table; width: 50%;
                     height: 50%; top: 25%; left: 25%">
      <div style="position: absolute; left: 0; right: 0; top: 0; bottom: 0;
                  background: green"></div>
    </fieldset>
  </body>
</html>