JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id="button">
        <div id="plus">+</div>
    </div>
</body>

CSS

#button {
    width : 100px;
    height : 100px;
    border-radius : 100%;
    background-color : red;
    display:table-cell;
    vertical-align: middle;
}
#plus {
    width : 50%;
    height : 50%;
    margin: auto;
    font-family : sans-serif;
    font-size : 40px;
    color : black;
    text-align: center;
}