JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div>Hello Plunker!</div>
    <div class="child">
        <div class="g-child"></div>
    </div>
</div>

CSS

html, body {
    height: 100%;
    padding: 0;
    margin: 0;
}

.parent {
    height:100%;
    display:flex;
    flex-direction: column;
    background-color:rgba(255,0,0,0.5);
}
.child {
    background-color:green; 
    flex-grow:1;
    height: 100%;
}

.g-child {
    background: blue;
    height: 70%; 
    width: 70%;

}