JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="a">
        a
    </div>
    <div id="b">
        b
    </div>
</div>

CSS

#container {
    width: 500px;
    border: solid 1px #000;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
}

#a {
    width: 20%;
    border: solid 1px #000;
}

#b {
    width: 20%;
    border: solid 1px #000;
    height: 200px;
    margin-left: auto;
}