[Excavator]

In your second example, your layout does not make 3 columns because #content_right is a child of #content_left. If you close #content_left before #content_right then #content_right can be beside #content_left instead of inside it.
Hope that was not too confusing.

Look at what an extra closing tag does -

                
                   </div>
  <div class="spacer"></div> <!-- note no content -->
</div>
    
</div>        
          <div id="content_right">
            <div class="full_sponsors">               </div>
            <!-- InstanceBeginEditable name="feature_section" -->