[javascript] javascript how to create a validation error message without using alert

JavaScript

<script language="javascript">
        var flag=0;
        function username()
        {
            user=loginform.username.value;
            if(user=="")
            {
                document.getElementById("error0").innerHTML="Enter UserID";
                flag=1;
            }
        }   
        function password()
        {
            pass=loginform.password.value;
            if(pass=="")
            {
                document.getElementById("error1").innerHTML="Enter password";   
                flag=1;
            }
        }

        function check(form)
        {
            flag=0;
            username();
            password();
            if(flag==1)
                return false;
            else
                return true;
        }

    </script>

HTML

<form name="loginform" action="Login" method="post" class="form-signin" onSubmit="return check(this)">



                    <div id="error0"></div>
                    <input type="text" id="inputEmail" name="username" placeholder="UserID" onBlur="username()">
               controls">
                    <div id="error1"></div>
                    <input type="password" id="inputPassword" name="password" placeholder="Password" onBlur="password()" onclick="make_blank()">

                    <button type="submit" class="btn">Sign in</button>
                </div>
            </div>
        </form>