我的问题！！！

<HTML>
<HEAD>
<TITLE>
Login.......
</TITLE>
<META CONTENT= "text/html; charset=gb2312" HTTP-EQUIV= "Content-Type">
<script language="JavaScript">
function checkform(Frm)
{
var flag=true;
if(Frm.uid.value=="")
{
alert("can't empty");
Frm.uid.focus();
flag=false;
}
return flag;
}
</script>
</HEAD>
<BODY BGCOLOR= "#FFFFFF">
<FORM ACTION= "main.jsp" METHOD= "post"  onsubmit="return checkform(this)">
<DIV ALIGN= "center">
<P>
&nbsp;
</P>
<P>
&nbsp;
</P>
<P>
&nbsp;
</P>
<P>
<INPUT NAME= "uid" TYPE= "text">
</P>
</DIV>
<P ALIGN= "center">
<INPUT NAME= "pass" TYPE= "password">
</P>
<P ALIGN= "center">
<INPUT VALUE= "Submit" TYPE= "Submit" NAME= "Submit">
</P>
</FORM>
</BODY>
</HTML>