JSFiddle - React, Tailwind, and code Playground

HTML

<p class="item">hello</p>

CSS

.item {
    opacity: 0.2;
}

.item.seen {
    opacity: 1;
    -webkit-transition: opacity 1s linear;
    -moz-transition: opacity 1s linear;
    transition: opacity 1s linear;
}

JavaScript

$('p').on('click', function() {
 $( this ).toggleClass('seen');
} );