html {
  font-size: 16px;
}
h1 {
  text-align: center;
}
img {
  max-width: 100%;
}
a,
a:visited,
a.hover {
  color: #333;
}
.box {
  display: block;
  border: 1px #333 solid;
  border-radius: 8px;
  margin: 8px;
  padding: 8px;
  text-align: center;
}
.box:hover {
  background-color: #ccc;
}
.box:visited {
  color: #333;
}

@media (min-width: 650px) {
  .flex {
    display: flex;
    flex-wrap: wrap;
  }
  .box {
    flex: 1;
    display: inline-block;
    min-width: 300px;
  }
}