JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<tr>
  <th id="th">TH</th>
</tr>
</table>

CSS

table {
  border-collapse: collapse;
}

#th {
  border-bottom: 11px solid red;
}

JavaScript

var th = document.getElementById('th');
var style = window.getComputedStyle(th);
console.log(style.getPropertyValue('border-bottom-width'));