JSFiddle - React, Tailwind, and code Playground

HTML

<a href="#">hover me to fade out main</a>
<div id="main">
</div>

CSS

#main {
    -webkit-transition: opacity 0.3s;
    -moz-transition: opacity 0.3s;
    -ms-transition: opacity 0.3s;
    -o-transition: opacity 0.3s;
    transition: opacity 0.3s;
    height:400px;
    width:400px;
    background:red
}

#main:not(:hover) {
    opacity: 0.3;
}

a:hover + #main {
    opacity:0.5;
}