1
Just a quick tip, in /xoopsheadline/templates/block/xoopsheadline_block_rss.html
change
<{foreach item=feed from=$block.feeds}>
<p><{$feed}>p>
<{/foreach}>
to
<{foreach item=feed from=$block.feeds}>
<br /><{$feed}><br />
<{/foreach}>
and you'll get a lot less XHTML errors in your main page.
The other source of Headlines related errors would be non-XHTML compliant tags embedded in RSS feeds, a tag sanitizer could solve this.