Untitled

<!DOCTYPE html>
    <html>
    <head>
    <meta charset="utf-8">
    <title>Hello, world!</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
    </script>
    <script>
        (function(){
            $('form > input').keyup(function(){
                var empty = false;
                $('form > input').each(function(){
                    if ($(this).val() == ''){
                        empty = true;
                    }
                });
                if (empty){
                    $('#submitForm').prop('disabled','disabled');
                } else {
                    $('#submitForm').prop('disabled',false);
                }
            });
        })()
    </script>
    </head>
    <body>
    <form method="POST">
        Username: <input type="text" name="username"><br>
        Password: <input type="password" name="password"><br>
        <input type="submit" id="submitForm" name="Log In" disabled="disabled">
    </form>
    </body>
    </html>