1
Just a quick tip, in /xoopsheadline/templates/block/xoopsheadline_block_rss.html
change
le="color: #000000"><?php <{foreach item=feed from=$block.feeds}> <p><{$feed}></p> <{/foreach}>
to
le="color: #000000"><?php <{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.