When you want to make your XOOPS xhtml-strict you have to watch out for a few things that differ from xhtml-transitional. One of these things is that target=blank is not allowed in xhtml-strict. I know that making XOOPS xhtml-strict is ahead of time, since XOOPS struggles to be xhtml-transitional, but I wanted to let you know. Add this function between the head-tags of your theme: Also there must be an onload of this function inside the body-tag, like: Also class/module.textsanitizer.php must be changed. Look for and replace with And this is the method how links that have to be opened in a new window must be written. Just add rel="external" to any link that has to open in a new window. I must confess that I don't know where the credits for the above snippet have to go to and how to place the function more elegant.