A simple no-JavaScript on-page-load modal message box

This is a simple CSS-only (no-JavaScript), on-page-load, modal message box. I adapted it from https://codepen.io/peiche/pen/vhqym by Paul @peiche

<style>
.forwarding-overlay {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: rgba(0, 0, 0, 0.5);
  transition: opacity 200ms;
  visibility: visible;
  opacity: 1;
  z-index: 1;
}
.forwarding-overlay .dismiss {
  position: absolute;
  width: 100%;
  height: 100%;
  cursor: default;
}
.forwarding-overlay:target {
  visibility: hidden;
  opacity: 0;
}
.forwarding-overlay .message-box {
  margin: 150px auto;
  padding: 20px;
  background: #fff;
  border: 1px solid #666;
  width: 800px;
  box-shadow: 0 0 50px rgba(0, 0, 0, 0.5);
  position: relative;
  color: #666;
}
.forwarding-overlay .message-box h2 {
  margin: 0 0 12px;
}
.forwarding-overlay .message-box .close {
  position: absolute;
  width: 24px;
  height: 24px;
  top: 20px;
  right: 20px;
  opacity: 0.8;
  transition: all 200ms;
  font-size: 24px;
  font-weight: bold;
  text-decoration: none;
  text-align: center;
}
.forwarding-overlay .message-box .close:hover {
  opacity: 1;
}
.forwarding-overlay .message-box .content {
  max-height: 400px;
  overflow: auto;
}
.forwarding-overlay .message-box a {
  color: blue;
  text-decoration: underline;
}
.forwarding-overlay .message-box a:hover {
  color: #00d;
}
.forwarding-overlay .message-box p {
  margin: 0 0 1em;
}
.forwarding-overlay .message-box p:last-child {
  margin: 0;
}
</style>
<div id="oldsite" class="forwarding-overlay">
  <a class="dismiss" href="#newsite"></a>
  <div class="message-box">
    <h2>This Website Has Moved</h2>
    <a class="close" href="#oldsite">X</a>
    <div class="content">
      <p>Thank you for visiting, but this site has moved/migrated.</p>
      <p>[old website name] is now located at <a href="https://new-website-address.tld/">https://new-website-address.tld/</a></p>
    </div>
  </div>
</div>