Volevo focalizzare il problema. Comunque ecco lo stesso documento (con lo stesso problema) validato XHTML-1.0:
Codice PHP:
<?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-IT" lang="it-IT">
<head>
<title>Prova</title>
<style type="text/css">
legend {
display: block;
width: 100%;
background-color: silver;
}
fieldset, form, body, html {
width: 100% !important;
}
</style>
</head>
<body>
<form action="http://www.chissenefrega.it" method="post">
<fieldset>
<legend>Titolo</legend>
<label>Un testo: <input type="text" /></label></p>
<input type="submit" value="CONFERMA" /></p>
</fieldset>
</form>
</body>
</html>