While I was testing the fmag.gr with the w3c validators, I saw several errors about
<h2> and <h5>
elements that where inside <a hrefs. I immediately spotted the problem on the node titles.
More specific: page views at fmag.gr are using fields. The field "node title" has a rewritten output to
<h5>[title]</h5>
and the "link to full node" checked.
Further investigation revealed that views module "links to full node" the whole "rewritten output" and not just the variable [title]. Therefore, the html output was
<h5><a href="...">[title]</a></h5>