body {
   max-width:800px;
   width: 90%;
   margin-left: auto;
   margin-right: auto;
}

h1 {
    font-size: 1.5rem;
}

@media (min-width: 640px) {
    body {
        background: #eaf6ff;
    }
    h1 {
        font-size: 2.5rem;
    }
    #stretchable-grid-items li{
        width: 25%;
    }
}

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


#fixed-size-grid-items ul{
    display: flex;
    flex-wrap: wrap;
    font-size: 0.8rem;
    padding: 0;
}

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

#stretchable-grid-items ul {
    display: flex;
    flex-wrap: wrap;
    font-size: 0.8rem;
    padding: 0;
    margin: 1rem -10px;
}

#stretchable-grid-items li{
    display: block;
    width: 100%;
}

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

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