html {
    width: 100%;
    height: 100%;
}

body {
    margin: 0;
    height: 100%;
    width: 100%;
}

body div {
  display: inline-block;
  height: 100vh;
  width: 50vw;
}

#one {
    background-color: red;
}

#two {
    background-color: blue;
}

@supports (display: flex) {
  body {
    display: flex;
  }
  
  body div {
    flex: 0 0 50%;
  }
}