#catalog .articles {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(18rem, 1fr));
  /* https://css-tricks.com/a-responsive-grid-layout-with-no-media-queries/ */
  gap: 2rem;
}

/* Bonus : hide all articles after the 3rd one until the user clicks the button to display more */
#catalog .only-display-6-articles article:nth-child(n+7) {
  display: none;
}