JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block1">
  1234
</div>
<div class="block2">
  1234
</div>

CSS

body {
    margin:0;
    display:flex;
    flex-direction:column;
    height:100vh;
}

.block1 {
    background-color:#ccc;
}

.block2 {
    flex:1 0 auto;
    background-color:#999;
}