html, body { margin: 0; padding: 0; }
body {
  width: min(800px, 90vw);
  margin: 0 auto;
}

@media (min-width: 640px) {
  body { background-color: #eaf6ff; }
}

h1 { line-height: 1.2; }
@media (max-width: 499px) {
  h1 { font-size: 1.5rem; }
}
@media (min-width: 641px) {
  h1 { font-size: 2.5rem; }
}

#fixed-size-grid-items ul {
  display: flex;
  flex-wrap: wrap;
  list-style: none;
  padding-left: 0;
  margin: 0;
}
#fixed-size-grid-items aside {
  width: 170px;
  margin: 10px;
  font-size: 0.8rem;
}

#stretchable-grid-items ul {
  display: flex;
  flex-wrap: wrap;
  list-style: none;
  padding-left: 0;
  margin: 1rem -10px;
}
#stretchable-grid-items li { width: 100%; }
@media (min-width: 481px) {
  #stretchable-grid-items li { width: 50%; }
}
@media (min-width: 641px) {
  #stretchable-grid-items li { width: 25%; }
}
#stretchable-grid-items aside { margin: 10px; }