JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<colgroup>
  <col>
  <col>
</colgroup>
<thead>
  <tr><th>1</th><th>2</th></tr>
</thead>
<tbody>
  <tr><td>hello</td><td>world</td></tr>
</tbody>
</table>

CSS

table{
width: 500px;
}

table > colgroup > col:first-child{
width: 30%;
}

JavaScript

alert($('table > colgroup > col:first').width());