<!--HTML-->
<form id='test_form' action='' method='' omsubmit='return checkForm()'>
    <input type='text' name='username' value=''/>
    <input type='password' name='pwd' value =''/>
    <button type='submit'>提交<button/>
<form/>

<script>
function checkForm(){
    var form = document.getElementById('test_form');
    //可在此修改input            
    //进行下一步
    form.submit();
}
</script>