Morphing Fullscreen Hamburger Menu With Pure HTML/CSS

A pure HTML/CSS side navigation that morphs the hamburger toggle button into a fullscreen nav menu using CSS3 transitions and transforms.

How to use it:

Wrap the menu items and toggle button into a label element as this:

<label>
  <input type="checkbox">
  <span class="menu">
    <span class="hamburger"></span>
  </span>
  <ul>
    <li>
      <a href="#">Home</a>
    </li>
    <li>
      <a href="#">About</a>
    </li>
    <li>
      <a href="#">Contact</a>
    </li>
  </ul>
</label>

Style the hamburger menu toggle.

label .hamburger {
  position: absolute;
  top: 135px;
  left: 50px;
  width: 30px;
  height: 2px;
  background: #000;
  display: block;
  -webkit-transform-origin: center;
  transform-origin: center;
  -webkit-transition: .5s ease-in-out;
  transition: .5s ease-in-out;
}
label .hamburger:after, label .hamburger:before {
  -webkit-transition: .5s ease-in-out;
  transition: .5s ease-in-out;
  content: "";
  position: absolute;
  display: block;
  width: 100%;
  height: 100%;
  background: #000;
}
label .hamburger:before { top: -10px; }
label .hamburger:after { bottom: -10px; }
label input { display: none; }
label input:checked + .menu {
  box-shadow: 0 0 0 100vw #FFF, 0 0 0 100vh #FFF;
  border-radius: 0;
}
label input:checked + .menu .hamburger {
  -webkit-transform: rotate(45deg);
  transform: rotate(45deg);
}
label input:checked + .menu .hamburger:after {
  -webkit-transform: rotate(90deg);
  transform: rotate(90deg);
  bottom: 0;
}
label input:checked + .menu .hamburger:before {
  -webkit-transform: rotate(90deg);
  transform: rotate(90deg);
  top: 0;
}
label input:checked + .menu + ul { opacity: 1; }

Style the fullscreen nav menu.

label .menu {
  position: absolute;
  right: -100px;
  top: -100px;
  z-index: 100;
  width: 200px;
  height: 200px;
  background: #FFF;
  border-radius: 50% 50% 50% 50%;
  -webkit-transition: .5s ease-in-out;
  transition: .5s ease-in-out;
  box-shadow: 0 0 0 0 #FFF, 0 0 0 0 #FFF;
  cursor: pointer;
}
label ul {
  z-index: 200;
  position: absolute;
  top: 50%;
  left: 50%;
  -webkit-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
  opacity: 0;
  -webkit-transition: .25s 0s ease-in-out;
  transition: .25s 0s ease-in-out;
}
label a {
  margin-bottom: 1em;
  display: block;
  color: #000;
  text-decoration: none;
}