img { 
    max-width: 100%; 
}

body{
    margin-left: auto; 
    margin-right: auto;
    width: 90%;
    max-width: 40rem;
}

@media (min-width: 640px) {
    html { font-size: 16pt; }
    .two-columns-example {display: flex;}
    .two-columns-example > * {width: 50%;}
}