JSFiddle - React, Tailwind, and code Playground

HTML

<table class="absenceTable">
    <thead>
        <tr>
            <th>Mitarbeiter</th>
            <th colspan="7">Oktober</th>
        </tr>
        <tr>
            <th>Zeit</th>
            <th><sub>1</sub></th>
            <th><sup>2</sup></th>
            <th><sub>3</sub></th>
            <th><sup>4</sup></th>
            <th><sub>5</sub></th>
            <th><sup>6</sup></th>
            <th><sub>7</sub></th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>Martina</td>
            <td></td>
            <td></td>
            <td><span>&nbsp;</span></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>
        <tr>
            <td>Werner</td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
            <td></td>
        </tr>		 
    </tbody>
</table>

CSS

table {
		  border-collapse: collapse;
		  border-spacing: 0;
		}

		table td, th {
		  border: 1px solid #999;	 
		  text-align: left;
		}
		table td {
		  padding:0; 
		  margin:0;
		}
		table span {
		  background-color: #f00;
		  border: none;
		}