JSFiddle - React, Tailwind, and code Playground

HTML

<hr />
<h1>Table-1</h1>
<hr />
<table class="tb1">
    <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td colspan="3">
            <table class="tb2">
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
            </table>

        </td>
    </tr>
     <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td colspan="3">
            <table class="tb2">
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
            </table>            
        </td>
    </tr>
</table>

<hr />
<h1>Table-2</h1>
<hr />

<table class="tb1">
    <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td colspan="3">
            <table class="tb2">
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
                <tr>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                    <td>tbrw2</td>
                </tr>
            </table>

        </td>
    </tr>
     <tr>
        <td>rw1</td>
        <td>rw1</td>
        <td>rw1</td>
    </tr>
    <tr>
        <td colspan="3">
            <table class="tb2">
                <tr>
                    <td>tbrw2</td>
                   ...

CSS

.tb1{
    border-collapse:collapse;
}

.tb1 tr td {
    border-bottom:1px solid #ccc;
    text-align:center;
    padding:10px;
}

.tb2{
    border-collapse:collapse;
}

.tb2 tr td {
    border-bottom:1px solid #f00;
    text-align:center;
    padding:10px;
}

JavaScript

$(function(){
    $('table.tb2').find('tr td').css({'border-bottom':'1px solid red'});
    $('table.tb2').find('tr:last td').css({'border-bottom':'none'});
    $('table.tb2').closest('td').css({'border-bottom':'none'});
});