main {
  float: left;
  width: 75%;
}

aside {
  float: right;
  width: 25%;
}

aside section {
  margin-left: 0;
}

/* stack main and aside on mobile */
@media (max-width: 840px) {
  main {
    width: 100%;
  }

  aside {
    width: 100%;
  }

  aside section {
    margin-left: 10px;
    margin-top: 0;
  }
}