Objectifs : permettre aux auteurs d' :
    
      - exprimer des contenus ou des ressources alternatives
- exprimer les conditions dans lesquelles chaque alternative doit
        être exploitée
<sel:select>
  <sel:when sel:expr="screen-width > 400 and available-colors > 4">
    <img alt="Many people had to be evacuated." src="imagebig"/>
  </sel:when>
  <sel:when sel:expr="screen-width > 100 and available-colors > 4">
    <img alt="Many people had to be evacuated." src="imagesmall"/>
  </sel:when>
  <sel:otherwise>
    <p>Many people had to be evacuated.</p>
  </sel:otherwise>
</sel:select>