JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/jquery.validate.js"></script>
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">
<table width="100%" border="0" cellspacing="0" cellpadding="0" id="tablecontent">
  <tr class="tablerow">
    <td>This is the td I want to add a class to.</td>
    <td class="cell2">Stuff</td>
    <td class="cell3">Stuff</td>
  </tr>
  <tr class="tablerow">
    <td>This is the td I want to add a class to.</td>
    <td class="cell2">Stuff</td>
    <td class="cell3">Stuff</td>
  </tr>
</table>

CSS

.someClass {
    background:yellow;
}

JavaScript

$('#tablecontent td:first-child').addClass('someClass');