If I write for example:
"Lets go to Moodle (http://moodle.org)."
and use the format option "Moodle auto-format", then the web address will be translated to:
<a href="http://moodle.org" target="_blank">http://moodle.org</a>
Notice the "target" parameter which is not allowed by XHTML 1.0 Strict.
I think the default behavior should be to strip the target parameter and to just let it open in the same window.
This issue has also been tested in the current http://demo.moodle.org/