Enforcing the separation of opening and closing tag in Lenya's HTML output

Hello,

I've got a question regarding the transformation of empty HTML tags in 
XSL files to the final HTML output produced by Lenya. I've got the 
following line in my page.xsl file:

<a class="addthis_button_facebook"></a>

The produced HTML then finally looks like this:

<a class="addthis_button_facebook"/>

The problem is that e.g. Safari has problems with interpreting that 
XHTML code correctly - thus leading to incorrect assignments of CSS 
styles (because of a missing closing tag).

I would expect that Lenya does not modify the HTML code, i.e. delivering

<a class="addthis_button_facebook"></a>

to the end user. Can you please tell me whether and how this could be 
achieved by configuration? Is there any workaround to enforce the 
separation of opening and closing tag?

Cheers
Christian