Presently, it is not possible to nest empty, non inline, custom tags in an XML Block field. Although this behavior seem to be intended, it doesn't seem to be coherent with what commonly happens in XHTML.

Steps to reproduce

• Configure the following custom tags (in content.ini.append.php)

  AvailableCustomTags[]=listsubitems
  AvailableCustomTags[]=tabbox
  
  IsInline[listsubitems]=false
  IsInline[tabbox]=false
  
  [listsubitems]
  CustomAttributes[]=main_node
  CustomAttributes[]=stylize
  CustomAttributes[]=limit
  CustomAttributes[]=children_view
  CustomAttributes[]=page
  

• Create an object, disable the Editor and enter the following conten

  <custom name="tabbox">
  <table class="default" width="100%" border="0" custom:summary="ncsare tabbox">
  <tr>
  <th width="0" custom:scope="col" custom:valign="top">Press Releases</th>
  <th width="0" custom:scope="col" custom:valign="top">Upcoming Events</th>
  </tr>
  <tr>
  <td>
  <custom name="listsubitems" custom:page="eznode://5821" custom:children_view="listitem" custom:limit="5" custom:stylize="nodate" custom:main_node="no"></custom>
  </td>
  <td>
  <custom name="listsubitems" custom:page="eznode://414" custom:children_view="listitem" custom:limit="5" custom:stylize="modular"></custom>
  </td>
  </tr>
  </table>
  </custom>
  

• Enable the editor
• If you disable the Editor again, you'll verify that the listsubitems tags have been removed