index.css

* {
  box-sizing: border-box;
}

body {
  font-family: sans-serif;
  margin: 0;
}

ul {
  list-style-type: none;
  padding: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-wrap: wrap;
  margin: 0;
}

h1 {
  color: #333;
}

a {
  display: inline-block;
  margin: 20px;
  width: 300px;
  max-width: 300px;
  max-height: 320px;
  overflow: auto;
  -webkit-box-shadow: 0 0 5px 1px rgba(155,155,155,0.2);
  box-shadow: 0 0 5px 1px rgba(155,155,155,0.2);
  transition: box-shadow 0.5s;
}

li a:hover,
li a:focus,
li a:active {
  box-shadow: 0 0 20px 1px rgba(5,5,5,0.2);
  transition: box-shadow 0.5s;
}

.container, a {
  width: 100%;
  height: 100%;
}

.container img {
  width: 100%;
  vertical-align: bottom;
}

main {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  flex-direction: column;
}

.heading {
  text-align: center;
}