set custom alert form validation

<form id="form" onsubmit="return(login())">
<input name="username" placeholder="Username" required />
<input name="pass"  type="password" placeholder="Password" required/>
<br/>Remember me: <input type="checkbox" name="remember" value="true" /><br/>
<input type="submit" name="submit" value="Log In"/>


var elements = document.getElementsByTagName("INPUT");
for (var i = 0; i < elements.length; i++) {
    elements[i].oninvalid = function(e) {
        e.target.setCustomValidity("");
        if (!e.target.validity.valid) {
            e.target.setCustomValidity("This field cannot be left blank");
        }
    };
    elements[i].oninput = function(e) {
        e.target.setCustomValidity("");
    };
}