onsubmlt阻止表单提交

<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<script language="javascript">
//--->
function checksubmit()
{
if (document.form1.myname.value=="")
{
alert("请输入开始数值！");
document.form1.myname.focus();
return false;
}
return true;
}
//--->
</script>

<form name="form1" method="post" action="#" onsubmit="return checksubmit()">
<label>
<input name="myname" type="text" id="id">
</label>
<p>
<label>
<input type="submit" name="Submit" value="提交">
</label>
</p>
</form>