Accessible Fullscreen Modal Popup with Pure CSS/CSS3

A responsive, fullscreen, fully accessible modal popup built with HTML/CSS/CSS3. All CSS. No JavaScript required.

How to use it:

Create the content for the modal, and use label, checkbox trick to launch a modal window.

<div class="modal">
  <input id="modal" type="checkbox" name="modal" tabindex="1">
  <label for="modal" >Launch Modal</label>
  <div class="modal__overlay">
    <div class="modal__box">
      <label for="modal">&#10006;</label>
      <h2>Modal Title</h2>
      <p>Modal Content</p>
    </div>
  </div>
</div>

The required CSS/CSS3 style rules for the modal popup.

.modal {
  display: block;
  padding: 0 1em;
  text-align: center;
  width: 100%;
}
@media (min-width: 43.75em) {
.modal {
  padding: 1em 2em;
  text-align: right;
}
}
.modal > label {
  background: #000;
  border-radius: .2em;
  color: #FFDE16;
  cursor: pointer;
  display: inline-block;
  font-weight: bold;
  margin: 0.5em 1em;
  padding: 0.75em 1.5em;
  -webkit-transition: all 0.55s;
  transition: all 0.55s;
}
.modal > label:hover {
  -webkit-transform: scale(0.97);
  -ms-transform: scale(0.97);
  transform: scale(0.97);
}
.modal input {
  position: absolute;
  right: 100px;
  top: 30px;
  z-index: -10;
}
.modal__overlay {
  background: black;
  bottom: 0;
  left: 0;
  position: fixed;
  right: 0;
  text-align: center;
  top: 0;
  z-index: -800;
}
.modal__box {
  padding: 1em .75em;
  position: relative;
  margin: 1em auto;
  max-width: 500px;
  width: 90%;
}
@media (min-height: 37.5em) {
.modal__box {
  left: 50%;
  position: absolute;
  top: 50%;
  -webkit-transform: translate(-50%, -80%);
  -ms-transform: translate(-50%, -80%);
  transform: translate(-50%, -80%);
}
}
@media (min-width: 50em) {
.modal__box { padding: 1.75em; }
}
.modal__box label {
  background: #FFDE16;
  border-radius: 50%;
  color: black;
  cursor: pointer;
  display: inline-block;
  height: 1.5em;
  line-height: 1.5em;
  position: absolute;
  right: .5em;
  top: .5em;
  width: 1.5em;
}
.modal__box h2 {
  color: #FFDE16;
  margin-bottom: 1em;
  text-transform: uppercase;
}
.modal__box p {
  color: #FFDE16;
  text-align: left;
}
.modal__overlay {
  opacity: 0;
  overflow: hidden;
  -webkit-transform: scale(0.5);
  -ms-transform: scale(0.5);
  transform: scale(0.5);
  -webkit-transition: all 0.75s cubic-bezier(0.19, 1, 0.22, 1);
  transition: all 0.75s cubic-bezier(0.19, 1, 0.22, 1);
}
input:checked ~ .modal__overlay {
  opacity: 1;
  -webkit-transform: scale(1);
  -ms-transform: scale(1);
  transform: scale(1);
  -webkit-transition: all 0.75s cubic-bezier(0.19, 1, 0.22, 1);
  transition: all 0.75s cubic-bezier(0.19, 1, 0.22, 1);
  z-index: 800;
}
input:focus + label {
  -webkit-transform: scale(0.97);
  -ms-transform: scale(0.97);
  transform: scale(0.97);
}