Basic Modal using only CSS

<div class="container">
  <div class="interior">
    <a class="btn" href="#open-modal"><i class="fas fa-external-link-alt"></i> Basic CSS-Only Modal</a>
  </div>
</div>
<div id="open-modal" class="modal-window">
  <div>
    <a href="#modal-close" title="Close" class="modal-close">Close</a>
    <h1>Voilà!</h1>
    <div>A CSS-only modal based on the :target pseudo-class. Hope you find it helpful. <a href="https://twitter.com/timothylong" target="_blank">Say hello on Twitter.</div>
    </div>
</div>

.modal-window {
  position: fixed;
  background-color: rgba(255, 255, 255, 0.25);
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index: 999;
  opacity: 0;
  pointer-events: none;
  transition: all 0.3s;
}
.modal-window:target {
  opacity: 1;
  pointer-events: auto;
}
.modal-window > div {
  width: 400px;
  position: absolute;
  top: 50%;
  left: 50%;
  -webkit-transform: translate(-50%, -50%);
          transform: translate(-50%, -50%);
  padding: 2em;
  background: #ffffff;
  color: #333333;
}
.modal-window header {
  font-weight: bold;
}
.modal-window h1 {
  font-size: 150%;
  margin: 0 0 15px;
  color: #333333;
}

.modal-close {
  color: #aaa;
  line-height: 50px;
  font-size: 80%;
  position: absolute;
  right: 0;
  text-align: center;
  top: 0;
  width: 70px;
  text-decoration: none;
}
.modal-close:hover {
  color: #000;
}

/* Demo Styles */
html,
body {
  height: 100%;
}

body {
  font: 600 18px/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
  background-color: #f69d75;
  color: #555555;
}

a {
  color: inherit;
}

.container {
  display: grid;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.btn {
  background-color: #fff;
  padding: 1em 1.5em;
  border-radius: 3px;
  text-decoration: none;
}
.btn i {
  padding-right: 0.3em;
}