I found out that Feedinformer adds an tag to the feed that destroys XHTML 1.0 Strict validation. Tag is:
div class="fdpoweredby"><img src="http://media.feed.informer.com/fd2P0ROYTYEWT.gif" alt="Provided by feed.informer.com" border="0" style="display:none"/></div>
I have use custom HTML templates. The problem lies in the border="0" part of the tag. A border definition is not allowed within an IMG tag with the XHTML 1.0 Strict DOM.
Any suggestions on how to go about this? The tag is not displayed (style="display:none"), so perhaps Feedinformer can delete it or at least remove the border="0" part.