codice:
<textarea cols="50" rows="100" onkeyup="if(this.value.length>0){document.getElementById('cancella').removeAttribute('disabled')}else{document.getElementById('cancella').setAttribute('disabled','disabled')} name="text" id="contact_text" class="inputbox required"></textarea>

<input type="reset" id="cancella" disabled="disabled" name="cancella" value="Cancella">