img { 
    max-width: 100%;
}

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

html {font-size: 12pt;}

@media (min-width: 640px) {
    html {font-size: 16pt;}
}

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