JSFiddle - React, Tailwind, and code Playground

HTML

<div id=fixed-height-container>
    <div id=horizontal-and-vertical-aligner>
        <div id=aligned-content>u sux at<br><span id=css>css</span></div>
    </div>
</div>

CSS

#fixed-height-container {
    display: table;
    width: 100%;
    height: 200px;
    background: green;
}

#horizontal-and-vertical-aligner {
    display: table-cell;
    background: lime;
    text-align: center;
    vertical-align: middle;
}

#aligned-content {
    display: inline-block;
    padding: 5px;
    border: 1px solid black;
    background: #fff;
}
#css { font-size: 2.5em; }