Grazie per la risposta tempestiva
Ma adesso (ho messo il charset), mi escono 2 errori
Ecco cosa dice il validator:

Result: Failed validation, 2 errors
File: index.html
Encoding: iso-8859-1
Doctype: XHTML 1.0 Strict
Root Namespace: http://www.w3.org/1999/xhtml

Note: The Validator XML support has some limitations.
This page is not Valid XHTML 1.0 Strict!

Below are the results of checking this document for XML well-formedness and validity.

1. Error Line 5 column 72: end tag for "meta" omitted, but OMITTAG NO was specified.

...ype" content="text/html; charset=iso-8859-1">

You may have neglected to close an element, or perhaps you meant to "self-close" an element, that is, ending it with "/>" instead of ">".


2. Info Line 5 column 0: start tag was here.

<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

3. Error Line 48 column 6: end tag for "div" omitted, but OMITTAG NO was specified.

</body>


4. Info Line 10 column 0: start tag was here.

<div id="corpo">


Ed ecco il mio codice :

<!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">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Layout a tre colonne con header e footer</title>
<link rel="stylesheet" type="text/css" href="stile.css" media="screen" />
</head>
<body>
<div id="corpo">
<div id="header">
<h1>MioSito.it</h1>
</div>

<hr />

<div id="colonna-sx"><div class="box">

<h3>Menu Verticale</h3>
Lorem ipsum...
</div></div>

<hr />

<div id="principale"><div class="box">
<h3>Contenuti</h3>


Lorem ipsum...</p>


Lorem ipsum...</p>


Lorem ipsum...</p>
</div></div>

<hr />

<div id="colonna-dx"><div class="box">
<h3>Adv</h3>
Lorem ipsum...
</div></div>

<hr />

<div id="footer">
Lorem ipsum...
</div>

</body>
</html>