Untitled

function validateForm() {
  var frm = document.login;
  if(frm.username.value=="abc" && frm.password.value=="abc123"){
      alert ();
  }else {
      alert ();
  }
}

<form id="login" name="login" action="" method="post" onsubmit="return validateForm()" >
   <h3>My Login Form</h3>
   <p>Username : <input type="text" name="username" id="username" /></p>
   <p>Password : <input type="password" name="password" id="password" /></p>
   <p><input type="submit" name="submit" id="submit" value="Login" /></p>
</form>

function validateForm() {
  var frm = document.login;
  if(frm.username.value=="abc" && frm.password.value=="abc123"){
      alert ();
  }else {
      alert ();
  }
}

<form id="login" name="login" action="" method="post" onsubmit="return validateForm()" >
   <h3>My Login Form</h3>
   <p>Username : <input type="text" name="username" id="username" /></p>
   <p>Password : <input type="password" name="password" id="password" /></p>
   <p><input type="submit" name="submit" id="submit" value="Login" /></p>
</form>

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <!-- Always force latest IE rendering engine (even in intranet) & Chrome Frame Remove this if you use the .htaccess -->
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
        <title>test2</title>
        <meta name="description" content="">
        <meta name="author" content="ITS The 1 - Yesh">
        <meta name="viewport" content="width=device-width; initial-scale=1.0">
        <!-- Replace favicon.ico & apple-touch-icon.png in the root of your domain and delete these references -->
        <link rel="shortcut icon" href="/favicon.ico">
        <link rel="apple-touch-icon" href="/apple-touch-icon.png">
        <script>
            function validateForm() {
              var frm = document.login;
              if(frm.username.value=="abc" && frm.password.value=="abc123"){
                  alert ("You Are Doing Well");
              }else {
                  alert ("Please Fill Your Username And Password!");
                  username.focus();
                  return false;
              }
            }
        </script>
    </head>
    <body>
        <div>
            <form id="login" name="login" action="" method="post" onsubmit="return validateForm()" >
               <h3>My Login Form</h3>
               <p>Username : <input type="text" name="username" id="username" /></p>
               <p>Password : <input type="password" name="password" id="password" /></p>
               <p><input type="submit" name="submit" id="submit" value="Login" /></p>
            </form>
        </div>
    </body>
</html>