JSFiddle - React, Tailwind, and code Playground

HTML

<table border="1" class="scrollableTable">
    <thead>
        <tr>
            <th width="14.2%">Codice RM</th>
            <th width="14.2%">Autore Firma</th>
            <th width="14.2%">Data Firma</th>
            <th width="14.2%">Acq Riserva</th>
            <th width="14.2%">Consegna Finale</th>
            <th width="14.2%">Descrizione RM</th>
            <th width="14.2%">Imponibile</th>
         </tr>
     </thead>

     <tbody>
         <tr class="even" id="rmRow">
             <td width="14.2%">0001</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">00000000</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">19/09</td>
             <td width="14.2%">57.0</td>
          </tr>
         
         <tr class="even" id="rmRow">
             <td width="14.2%">0002</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">00000000</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">19/09</td>
             <td width="14.2%">57.0</td>
         </tr>
         
         <tr class="even" id="rmRow">
             <td width="14.2%">0003</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">00000000</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">19/09</td>
             <td width="14.2%">57.0</td>
          </tr>
         
         <tr class="even" id="rmRow">
             <td width="14.2%">0004</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">00000000</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">bla</td>
             <td width="14.2%">19/09</td>
             <td width="14.2%">57.0</td>
         </tr>
         
         <tr class="even" id="rmRow">
             <td width="14.2%">0005</td>
             <td width="14.2%">bla</td>
             <td...

CSS

tbody {
        margin-top: 40px;
    }

    table {
        display: block;
        height: 200px;
        overflow-y: scroll;
        position: relative;
        margin-top: 50px;
    }
    thead {
        background: white;
        position: fixed;
        top: 0;
        margin-right: 25px;
        display: table-row-group;
    }
    th, td {
        width: 80px;
    }

JavaScript

var tbody = document.getElementsByTagName('tbody')[0];