il tuo codice è diverso da questo?

codice:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" 
      xml:lang="it" 
      lang="it">

<head>

   <link rel="stylesheet" 
         type="text/css" 
         href="stile.css" />

</head>