Issue with jquery validation plugin on IE9

Issue with jquery validation plugin on IE9

Hello all.
This is my first post on the board and comes from an afternoon of exasperation.
I have a form that needs to send data, I installed the plugin jquery validation, which works perfectly on chrome, on firefox and IE8 ... but does not work on IE9.
The code is as follows:

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />

  <script src="js/jquery-1.8.0.min.js"></script>
   <script type="text/javascript" src="http://jzaefferer.github.com/jquery-validation/jquery.validate.js"></script>
   <script src="js/html5.js"></script>

 <form action="" id="formDocumentazione" method="post">
            <fieldset id="dettagli-utente">
              <div style="width: 20%; float:left">
                <label for="cognome">Cognome:</label>
              </div>
              <div style="width:80%; float:left">
                <input type="text" name="cognome" value="" class="required" />
              </div>
              <div style="width: 20%; float:left">
                <label for="nome">Nome:</label>
              </div>
              <div style="width:80%; float:left">
                <input type="text" name="nome" value="" class="required" />
              </div>

This is just a piece of code but is used to make it work in theory.
But in IE9 the plugin does not feel validation, and continues with the fields completely empty.
I've tried several solutions without success.
I need your help.
Thanks