<div class="subscribe subscribe-1">
  <a href="#" class="subscribe__toggle" id="toggle">Sign up for the Beta</a>
  <span class="subscribe__success">You're all in set, check your email.</span>
  <div class="subscribe__wrapper">
    <input type="text" />
    <button id="submit">Sign Up</button>
  </div>
</div>
<a href="https://codepen.io/collection/DpgWyG" class="more-concepts" target="_blank">See more in this collection</a>

html {
  box-sizing: border-box;
  font-size: 100%;
  font-family: Arial;
}

* {
  box-sizing: inherit;
}

$blue: #358ecc;

.subscribe-1 {
  position: absolute;
  left: 0; top: 0; right: 0; bottom: 0;
  width: 25.25em;
  height: 5em;
  margin: auto;
  padding: 1em 0.75em;
  border: 3px solid transparent;
  border-radius: 8px;
  overflow: hidden;
  transition: width 0.15s ease;
  
  &__active {
    width: 31.25em;
    border: 3px solid $blue;
  }
  
  &__success {
    border: 3px solid #4cce95;
  }
  
  input[type="text"] {
    color: $blue;
    width: 75%;
    height: 100%;
    margin-right: 2%;
    padding-left: 0.5em;
    padding-right: 0.5em;
    font-size: 1rem;
    border: 0;
    outline: 0;
  }
  
  button {
    flex-grow: 1;
    border: 0;
    border-radius: 4px;
    background: darken($blue, 10%);
    color: #fff;
    cursor: pointer;
    font-size: 1em;
    transition: background 0.2s ease;
    
    &:hover {
      background: $blue;
    }
  }
}

.subscribe__wrapper {
  height: 100%;
  display: flex;
}

.subscribe__toggle {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  background: darken($blue, 10%);
  text-align: center;
  text-decoration: none;
  color: #fff;
  font-size: 1.25em;
  line-height: 70px;
  border-radius: 4px;
  transition: opacity 0.15s ease, background 0.2s ease;
  //display: none;
  
  &__hidden {
    opacity: 0;
    pointer-events: none;
  }
  
  &:hover {
    background: $blue;
  }
}

.subscribe__success {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  line-height: 70px;
  color: #4cce95;
  text-align: center;
  background: #fff;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.25s 0.2s ease;
  
  &--active {
    opacity: 1;
  }
}

.more-concepts {
  position: absolute;
  left: 50%;
  bottom: 20px;
  transform: translateX(-50%);
  text-decoration: none;
  color: grey;
  
  &:hover {
    text-decoration: underline;
  }
}

var toggle = document.getElementById('toggle'),
  wrapper = document.querySelectorAll('.subscribe'),
  submit = document.getElementById('submit'),
  success = document.querySelectorAll('.subscribe__success'),
  content = document.querySelectorAll('.subscribe__wrapper');

toggle.addEventListener('click', function() {
  this.className += ' subscribe__toggle__hidden';
  wrapper[0].className += ' subscribe-1__active';
});

submit.addEventListener('click', function() {
  success[0].className += ' subscribe__success--active';
  wrapper[0].className += ' subscribe-1__success';
  content[0].style.display = 'none';
});