<div id="div1"></div>
<div id="div2"></div>

body {
    width: 100%;
    height: 100vh;
    margin: 0px;
}
#div1 {
    background-color: blue;
    height: 100%;
    width: 100%;
}
#div2{
    background-color: red;
    height: 100%;
    width: 100%;
}