表单不能提交

不清楚。看例子：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>test</title>
<script language="javascript">
function addObj()
{
var ipt=document.createElement("INPUT");
ipt.setAttribute("name","mytext");
ipt.setAttribute("type","text");
ipt.setAttribute("size",20);
document.form1.insertAdjacentElement("AfterBegin",ipt);
var txt=document.getElementsByTagName("INPUT");
alert(txt.length-2);
}
function checkForm()
{
var txt=document.getElementsByTagName("INPUT");
var rt=true;
for(i=0;i<txt.length-2;i++)
{
if(txt[i].value=="")
{
alert("mytext["+i+"].value is empty");
rt=false;
break;
}
}
return rt;
}
</script>
</head><body>
<form name="form1" method="post" action="abc.htm" onsubmit="return checkForm();">
  <input name="mytext" type="text" size="20" />
  <input name="button" type="button" value="新增" onclick="addObj()"/>
  <input type="submit" name="Submit" value="提交" />
</form>
</body>
</html>