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" lang="it" xml:lang="it">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
        
        <title>Esempio</title>
        
        <script type="text/javascript">
        // <![CDATA[
        b_open = false; 
        
        function addBold(ta, btt) {
            el_ta = document.getElementById(ta);
            if (b_open) {
                el_ta.value += "[/b]";
                btt.value = "[b]"
            }
            else {
                el_ta.value += "";            
                btt.value = ""
            }
            
            b_open = !b_open;
        }
        // ]]>
        </script>
    </head>

<body>

  <textarea id="testo"></textarea>
  
  <input type="button" value="[b]" onclick="addBold('testo', this);"/>

</body>
</html>

-Fab-