body {
    margin: auto;
    width: 80%;
    max-width: 800px;
}   

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

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

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

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

#fixed-size-grid-items li{
    list-style-type: none;
}

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

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

#stretchable-grid-items li{
    width: 100%;
    list-style-type: none;
}

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

@media (width>640px){
    #stretchable-grid-items li{
        width: 25%; 
    }
}