JavaScript in Signin.html page not working in Safari & Androd browsers

Hi all
We have customized signin.html as the following using a JavaScript.

1.  Added a checkbox which says 'I have read and accept Terms and Conditions'
2.  If the user does not check that box, then he cannot login. I am implementing this functionality using java script. 

This is not working in Safari/Android Browsers.. In Safari/Browser, user can login without checking the box...

Any ideas are appreciated.

function submitAction(form)
{  
if (document.getElementById("chk_terms").checked )
    {
signin(form);
form.Submit.disabled=true;
form.submit();
 }
else
 {
    form.userid.value = "";
    form.pwd.value = "";   
    alert("<%=255%>");
    return false;
 }
}


thanks
Rahul