JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper one">
    ...
    <div id="a">Div A</div>
    ...
</div>

<div class="wrapper two">
    ...
    <div id="b">Div B</div>
    ...
</div>

CSS

body {
    padding: 10%;
}

#a:hover #b {
    background: #ccc;
}

.wrapper {
    float:left;
}

.wrapper.one {
    width: 80%;
    font-size: 1.2em;
}

.wrapper.two {
    width: 20%;
    font-size: 0.8em;
}