css window height

div {
    height: 100vh;
}

body, html {
  height: 100%;
}

#right {
  height: 100%;
}

.left {
  height: 100vh;
  width: 50%;
  background-color: grey;
  float: left;
}

.right {
  height: 100vh;
  width: 50%;
  background-color: red;
  float: right;
}