body {
    margin: 0;
    padding: 0;
    background-color: darkgray;
  }

html {
    font-size: 16px;
}
@media screen and (min-width: 900px) {
    html {
        font-size: 18px;
    }
}

@media screen and (min-width: 1200px) {
    html {
        font-size: 20px;
    }
}

div { box-sizing: border-box; }