body {
  width: min(90%, 800px);
  margin: auto;          
}

@media (min-width: 640px) {
  body{ background-color: #eaf6ff; }
  h1 { font-size: 2.5rem; } 
}

@media (max-width: 500px) {
    h1 { font-size: 1.5rem; } 
}

#fixed-size-grid-items ul {
  list-style: none;
  padding-left: 0;
  display: flex;          
  flex-wrap: wrap;
}

#fixed-size-grid-items aside {
  width: 170px;
  margin: 10px;
  font-size: 0.8rem;
}

#fixed-size-grid-items li { 
    display:flex;
 }


#stretchable-grid-items ul {
  list-style: none;
  padding-left: 0;
  margin: 1rem 0;   
  margin-left:-10px;
  margin-right:-10px;
  margin-top:1rem;
  margin-bottom:1rem;      
  display: flex;
  flex-wrap: wrap;
}

#stretchable-grid-items li {
  width: 100%;
}
@media (min-width: 480px) {
  #stretchable-grid-items li { width: 50%; }
}
@media (min-width: 640px) {
  #stretchable-grid-items li { width: 25%; }
}

#stretchable-grid-items aside {
  margin: 10px;
}