<script type="text/javascript">
function prefillClear(field) {
if (field.defaultValue==field.value) {field.value = '';}
else if (field.value == '') {field.value = field.defaultValue;}
}
</script>

<!--HTML-->
<input type="text" name"cocodenut" value="prefilled text" onfocus="prefillClear(this);" onblur="prefillClear(this);"/>