img { max-width: 100%; }

body{
    margin-left: auto; 
    margin-right: auto;
    width: 90%;
    /* upper width limit of 40rem for content */
    max-width: 40rem; 
}

html { font-size: 12pt; }
/* when window width is at least 640px */
@media (min-width: 640px) {
    html { font-size: 16pt; }
}

@media (min-width:640px){
    .two-columns-example{
        /* set these 2 containers side by side when theres at least 640px of available width */
        display:flex; 
    }

    /* select both children of .two-columns-example */
    .two-columns-example > *{ 
        width: 50%;
    }
}