CSS Lightbox

index.html

<a href="#unique-id-of-image">
  <img alt="alt text" src="../assets/filename.png" />
</a>

<a href="#" class="lightbox" id="unique-id-value">
  <img alt="alt text" src="../assets/filename.png" />
</a>

style.css

.lightbox {
  display: none;
  position: fixed;
  justify-content: center;
  align-items: center;
  z-index: 999;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  padding: 1rem;
  background: rgba(0, 0, 0, 0.8);
}

.lightbox:target {
  display: flex;
}

.lightbox img {
  max-height: 100%
}