Javascript :: Login Page 'if' Display Something In HTML

I am creating a login page in HTML with Javascript Elements.

Basically I have a login form. and if credentials are wrong I want it to show an alert in HTML which another Javascript manages. Anyway I have setup a test page here As you can see when you input the wrong credentials it will display an alert. But I want it to perform an action in HTML? (This is so hard to describe)

<div class="notification error"> That is the code in HTML which will show that red box you see up the top & I will add text later.

Here's the Login form:

else {
$('.notification-detials')
}
return true;

[Code].....