html, body, .box {
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
}

.box {
    display: flex;
    flex-flow: column;
}

header {
    margin-bottom: 1em;
}

section {
    flex-grow: 1;
    width: 100%;
    height: 100%;

    display: flex;
    flex-flow: column;
}

section iframe {
    flex-grow: 1;
    width: 100%;
    height: 100%;
    border: none;
}

footer {
    min-height: 1em;
}

/* small (phone) */
@media screen and (min-width: 35.5em) {

}

/* medium (tablet) */
@media screen and (min-width: 48em) {

}

/* large (desktop) */
@media screen and (min-width: 64em) {

}

/* extra large (widescreen) */
@media screen and (min-width: 80em) {

}