JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li data-relation_id="1">a</li>
    <li data-relation_id="1">b</li>
    <li data-relation_id="1">c</li>
    <li data-relation_id="2">d</li>
    <li data-relation_id="2">e</li>
    <li data-relation_id="2">f</li>
    <li data-relation_id=3>g</li>
    <li data-relation_id="3">h</li>
    <li data-relation_id="3">i</li>
</ul>

JavaScript

$('li[data-relation_id="3"]').css('background-color', '#88F');