[CSS] Create an Automatically Responsive Flexbox Image Gallery

With just a handful of CSS properties, we can create an intrinsically responsive photo gallery using flexbox. This is accomplished by setting our preferred width as the flex-basis value and allowing items to both shrink and grow. The use of object-fit: cover on images allows them to fully fill up the list item without distortion or overflow. We finish the gallery with a small animated effect that alters opacity.

body {
  margin: 0.5rem;
  background-color: #222;
}

.gallery {
  list-style: none;
  padding: 0;
  margin: 0;

  display: flex;
  flex-wrap: wrap;
}

.gallery li {
  flex: 1 1 15rem; /*flex-grow and flex-shrink flex-baisis*/
  min-height: 30vh;
  max-height: calc(50vh - 0.5rem);
}

img {
  object-fit: cover;
  width: 100%;
  height: 100%;

  opacity: 0.7;
  transition: 180ms opacity ease-in-out;
}

img:hover {
  opacity: 1;
}