Una cosa cosi ti puo andare bene?

Codice PHP:
<!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=utf-8"/>
        <title>Prova</title>
        <script>
            function conta(oggetto)
            {                
                var inputcaratteri=document.getElementById("caratteri");
                var mioinput=document.getElementById("testo");
                var carattere="a";
                var quanti;
                quanti=0;
                for (indice=0;indice<=mioinput.value.length-1;indice++)
                {
                    if(carattere==mioinput.value.charAt(indice))
                    {
                        quanti=quanti+1;
                    }
                }
                inputcaratteri.value=quanti;
            }
        </script>
    </head>
    <body>
        <table>
            <tr>
                <td>TESTO
                </td>
                <td>
                <input onkeypress="conta(this)" style="margin-left:20px" type="text" id="testo" name="testo" />

                </td>
            </tr>
            <tr>
                <td>
                    SPAZI
                </td>
                <td>
                    <input readonly style="margin-left:20px" type="text" id="caratteri" name="caratteri" />

                </td>
            </tr>
            <tr>
                <td colspan="2">
                    <input type="button" value="conta" onclick="conta()" />
                </td>                
            </tr>
        </table>
    </body>
</html>