JSFiddle - React, Tailwind, and code Playground

HTML

<div id="div1">
    <div id="inner1">
    </div>
    <div id="inner2">
    </div>
</div>

CSS

#div1 {
    width: 500px;
    background-color: red;
    height: 500px;
}

#inner1 {
    width: 40%;
    display: inline-block;
    height: 100%;
    background-color: blue;
}
#inner2 {
    width: 60%;
    display: inline-block;
    height: 100%;
    background-color: green;
}