See https://github.com/nicholaskajoh/Simple-CSS-Grid.

grid.css

main {
  max-width: 1200px;
  margin: 0 auto;
}
section {
  display: flex;
  flex-wrap: wrap;
}
article, aside { flex-basis: 100%; }
@media (min-width: 576px) {
  article { flex: 3; }
  aside { flex: 2 }
}
@media (min-width: 768px) {
  article { flex: 7; }
  aside { flex: 3 }
}
@media (min-width: 992px) {
  article { flex: 1.5; }
  aside { flex: 0.5 }
}