<select name="sel">
<option value="1">1</option>
<option value="2">2</option>
</select>
<script>
function check(){
if(document.XXX.sel.value=="" || document.XXX.sel.value==null)
return;
}
</script>
<option value="1">1</option>
<option value="2">2</option>
</select>
<script>
function check(){
if(document.XXX.sel.value=="" || document.XXX.sel.value==null)
return;
}
</script>
<%
i=1
if i=1 then%>
<option value="1" selected>1</option>
<%
i=i+1
end if%>
<option value="2">2</option>
</select>
<option>
<option value="1">1</option>
<option value="2">2</option>
</select>
<p><input type=button value=submit onclick="if(s1.value == '') alert('请选择一项!');"></p>
<select>
<option value="1" selected>...</option>
</select>
<select name=s1>
<option>
<option value="1">1</option>
<option value="2">2</option>
</select>
<p><input type=button value=submit onclick="if(document.getElementById("s1").value == '') alert('请选择一项!');"></p>