Prevent XSS using DOM Extension and/or SimpleXML

Hi there! I´m building a form validator using PHP and JS. It´s working fine by now, but I want to make a little improvement. Here is how its working now: 1. The user fill the form. Every time he leaves the field, the JS

    code match the value against a regexp to validate.

2. When the user submits the form, the PHP script match all the

    values against the same regexp's.

Now, i want to validate my fields to prevent XSS, allowing my html tags but only the attributes that I want. I thought something like: (the tags and the valid attributes). <?php $form_html_validation = array(

      "p"=>array(""),
      "a"=>array("href","name","rel"),
      "ol"=>array(""),
      "ul"=>array(""),
      "li"=>array(""),
      "h2"=>array(""),
      "h3"=>array(""),
      "h4"=>array(""),
      "h5"=>array(""),
      "h6"=>array(""),
      "strong"=>array(""),
      "em"=>array("")  );

$valid_elements = "<".join("><",array_keys($form_html_validation)).">"; $userInput = strip_tags($userInput,$valid_elements); //perform DOM Attribute Validation ?> But I don´t know how to loop over every attribute for each tag in the DomTree. Someone has any ideas? Thank You