JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
  <head>
    <title>Sample</title>
  </head>

  <body>
    <br><br><br><br>

    <table id="A" border="0" width="95%" cellspacing="0" cellpadding="0" align="center" class="base">
      <tr bgcolor='gray'>
        <td>
          <br><br><br>
          need the blue column heading rows to remain fixed, and scroll the green rows:<br>

    <table id="XYZ" border="1" width="625" cellspacing="0" cellpadding="0" align="center" class="base">
      <thead>
        <tr>
          <th width="50px" bgcolor="DeepSkyBlue" align="center" valign="middle">Col 1a</th>
          <th width="50px" bgcolor="DeepSkyBlue" align="center" valign="middle">Col 1b</th>
          <th width="75px" bgcolor="DeepSkyBlue" align="center" valign="middle">Col 1c</th>
          <th width="100px" style="border-left:medium solid black;" colspan="3" bgcolor="DeepSkyBlue" align="center" valign="middle"><b>Col 2</th>
          <th width="100px" style="border-left:medium solid black;" colspan="1" bgcolor="DeepSkyBlue" align="center" valign="middle"><b>Col 3</th>
          <th width="150px" style="border-left:medium solid black;" colspan="5" bgcolor="DeepSkyBlue" align="center" valign="middle"><b>Col 4<br>more<br>more</th>
          <th width="100px" style="border-left:medium solid black;" colspan="1" bgcolor="DeepSkyBlue" align="center" valign="middle"><b>Col 5</th>
        </tr>
        <tr>
            <th bgcolor="DeepSkyBlue" colspan="3" align="center" valign="middle">Col 1</th>
            <th bgcolor="DeepSkyBlue" style="border-left:medium solid black;" align="center" valign="middle">A</th>
            <th bgcolor="DeepSkyBlue" align="center" valign="middle">B</th>
            <th bgcolor="DeepSkyBlue" align="center" valign="middle">C</th>
            <th bgcolor="DeepSkyBlue" style="border-left:medium solid black;" align="center">1</th>
            <th bgcolor="DeepSkyBlue" style="border-left:medium...

CSS

thead{position:fixed;top:20px;width:auto;}