A simple responsive grid using flexbox

simple_flex_grid.css

.grid .cell {
  position: relative;
  margin: 1rem 1rem 2rem;
}

@media screen and (min-width: 600px) {
  .grid {
    display: flex;
    flex-wrap: wrap;
    flex-direction: row;
  }
  .grid .cell {
    width: calc(50% - 2rem);
  }
}

@media screen and (min-width: 1000px) {
  .grid .cell {
    width: calc(33.3333% - 2rem);
  }
}