JSFiddle - React, Tailwind, and code Playground
HTML
<div class="des">
<h1>Test</h1>
<div class="desleft">
<p>Lorem Ipsum.</p>
</div>
<div class="Right">
<button>Test</button>
</div>
</div>
JavaScript
alert(document.getElementsByClassName("desleft")[0].previousSibling.previousSibling.tagName)