JSFiddle - React, Tailwind, and code Playground

HTML

<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 />
            <div class="fTable_container">
                <div class="fTable_rContainer">
            <table id="XYZ_fixed" border="1" width="625" cellspacing="0" cellpadding="0" align="center" class="base fTable_fixedHead">
            <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>
               ...

CSS

.fTable_rContainer { 
    position: relative; 
    overflow: auto;    
    height: 300px;
}

.fTable_container {
    width: 643px;
}

.fTable_fixedHead {
    position: fixed;;    
}