JSFiddle - React, Tailwind, and code Playground

HTML

<div class="test px"></div>

<per id="_log"></per>

CSS

.test {
  border-top: 3px solid blue;
  height: 10px;
  width: 96px;
}

JavaScript

const CSSpxTocm = (CSSpx) => CSSpx * 2.54 / (96 / devicePixelRatio);

_log.textContent =CSSpxTocm(96) + 'cm';