JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a"><div class="b"></div></div>

<br>
<br>
    
<div class="a"><div class="c"></div></div>

CSS

.a {
    background: blue;
}

.b {
    display:inline-block;
    height: 30px;
    width: 30px;
    background: red;
}

.c {
    display:block;
    height: 30px;
    width: 30px;
    background: red;
}