JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Table-1</h1>

<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>tbrw21</td>
                    <td>tbrw21</td>
                    <td>tbrw21</td>
                </tr>
                <tr>
                    <td>tbrw22</td>
                    <td>tbrw22</td>
                    <td>tbrw22</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 {
    text-align:center;
    padding:10px;
}

JavaScript

$(".tb2 tr:first-of-type").css("border-bottom","2px solid red");