.feed {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  align-items: stretch;
  max-width: 800px;
  margin: 0 auto;
}

.post {
  width: calc(25% - 1em);
  margin-bottom: 1em;
  position: relative;
}

.post img {
  width: 100%;
  height: auto;
}

.post:before {
  content: "";
  display: block;
  padding-top: 100%;
}

.post a {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
}

@media screen and (max-width: 800px) {
  .post {
    width: calc(33% - 5px);
  }
}

@media screen and (max-width: 480px) {
  .post {
    width: 25%%;
  }
}