New Test

HTML

<div id="test-form">
  <h2>Add a User:</h2>
  <label for="Username:">
   <input id="username" type="text" name="username" placeholder="Type username here">
 </label>
  
  <label for="Email:">
  <span id='result'></span>
  <input id="email" type="email" name="email" placeholder="email">
  </label>
  <button id="myButton" onclick="myTest()">add user</button>
  <h2 class="clear">Users:</h2>
  <ul id="users"></ul>
</div>

CSS

@import url('https://fonts.googleapis.com/css?family=Roboto+Condensed');

* {
   box-sizing:border-box;
   -webkit-box-sizing:border-box;
   -moz-box-sizing:border-box;
   -webkit-font-smoothing:antialiased;
   -moz-font-smoothing:antialiased;
   -o-font-smoothing:antialiased;
   font-smoothing:antialiased;
   text-rendering:optimizeLegibility;
}

body
{

  font-family: 'Roboto Condensed', sans-serif;
  background: #59ABE3;
}
#test-form
{
    width: 340px;
    min-height: 440px;
    background: #e6e6e6;
    border-radius: 8px;
    box-shadow: 0 0 40px -10px #000;
    margin: calc(50vh - 220px) auto;
    padding: 20px 30px;
    max-width: calc(100vw - 40px);
    box-sizing: border-box;
    font-family: 'Montserrat',sans-serif;
    position: relative;
    border-radius:7px;
    -webkit-border-radius:7px;
    -moz-border-radius:7px;
}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

li {
  background: #ffc0cb;
  padding: 5px;
  margin: 4px;
}

h2 {
    margin: 10px 0;
    padding-bottom: 10px;
    width: 180px;
    color: #78788c;
    border-bottom: 3px solid #78788c;
}

label {
  display: block;
}

label:before {
  content:attr(for);
  display:block;
  margin:28px 0 0;
  font-size:14px;
  color:#5a5a5a
  }

#test-form input {
  width:100%;
  padding:10px;
  box-sizing:border-box;
  background:none;
  outline:none;
  resize:none;
  border:0;
  font-family:'Montserrat',
  sans-serif;
  transition:all .3s;
  border-bottom:2px solid #bebed2
  }

#test-form input:focus {
  background: #ffed97;
  border-bottom:2px solid #78788c
  }

span#result {
    color: #ca0303;
    font-size: 14px;
    display: -webkit-inline-box;
    padding-top: 8px;
}

button#myButton {
  float:left;
  padding:8px 12px;
  margin:8px 0 19px;
  font-family:'Montserrat',sans-serif;
  border:2px solid #78788c;
  background:0;
  color:#5a5a6e;
  cursor:pointer;
  transition:all .3s
  }
  
  .clear {
    clear: left;
  }
  
button#myButton:hover {
  background:#78788c;
  color:#fff
 ...

JavaScript

function myTest() {
  addUser(username, email, onSuccess);
}

function onSuccess(result){
  var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
 
  var username = document.getElementById("username").value;
  var email = document.getElementById("email");
	var message = document.getElementById("result").classList;
   
  document.getElementById("users").innerHTML+= '<li>' + username +'</li>';

if (!filter.test(email.value)) {
    document.getElementById('result').innerHTML+='Not a valid email';
    email.focus;
     return false;
 } else {
 
 //I could have cleared the contents of the 'result' span by using something similar to document.getElementById('result').innerHTML=' ' But I figured I'd have more control manipulating the span results using CSS. In this situation we may be better off using a Javascript library like prototype or jQuery, which have methods to do this, as well as functions that can first check if an element already has a class assigned.
 
    message.add("hide");
  }
 
}



// Do not modify this function. Add user service wrapper.
function addUser(username, email, callback) {
    var xhr = new XMLHttpRequest();
    var response;
    var success = (!!Math.round(Math.random()));
    
    if (!success){
        response = JSON.stringify({
            success: success,
            error: "Oups, something went wrong!"
        });
    } else {
        response = JSON.stringify({
            success: success,
            user: {
                username: username,
                email: email
            }
        });   
    }
    
    xhr.open("POST", "/echo/json/");
    xhr.onload = function () {
    		if (xhr.status === 200) {
        		callback(JSON.parse(xhr.responseText));
        }
    }
    xhr.send("json=" + response);
};