body{margin-left: auto;
    margin-right: auto;
    max-width: 90%;
    width: 800px; 
text-align: center;}
h1{font: size 1.5rem;}
#fixed-size-grid-items ul {display: flex;
    flex-wrap: wrap;
    font-size: 0.8rem;
    padding: 0;}
#stretchable-grid-items li{
    width: 25%;
    display:block;}
#stretchable-grid-items ul{
    display: flex;
    flex-wrap: wrap;
    font-size: 0.8rem;
    padding: 0;}
#fixed-size-grid-items li{
    display: block;}
#fixed-size-grid-items aside{
    margin: 10px;
    width:170px;}
@media (min-width: 640px) 
{body{background: #eaf6ff;}
    h1{font-size: 2.5rem;}


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