JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="child1">
        <h1>Text</h1>
    </div>
    <div class="child2">
        should be centered vertically
    </div>
</div>

CSS

.container {
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
    
    justify-content: space-between;
    align-items: center;
    background-color: yellow;
}

.child1 {
    background-color: green;
}

.child2 {
    border-left: 1px black solid;
    align-self: stretch;
}