JSFiddle - React, Tailwind, and code Playground

HTML

<table>
		 <thead>
		  <tr>
			<th>Mitarbeiter</th>
			<th colspan="11" class="center first">Oktober</th>
			<th colspan="11" class="center first">November</th>
		  </tr>
		  <tr>
			 <th>Zeitskala</th>
			 <th class="first"><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>
			 <th><sup>8</sup></th>
			 <th><sub>9</sub></th>
			 <th><sup>10</sup></th>
			 <th><sub>11</sub></th>
			 
			 <th class="first"><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>
			 <th><sup>8</sup></th>
			 <th><sub>9</sub></th>
			 <th><sup>10</sup></th>
			 <th><sub>11</sub></th>
		  </tr>
		 </thead>
		 <tbody>
		  <tr>
			 <td>Martina</td>
			 <td class="first"></td>
			 <td></td>
			 <td><span>&nbsp;</span></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 
			 <td class="first"></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
		  </tr>
		  <tr>
			 <td>Werner</td>
			 <td class="first"></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 <td></td>
			 
			 <td class="first"></td>
			 <td></td>
			 <td></td>
			 <td><span>&nbsp;</span></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 tr th {
    min-width: 10px;
}
table td {
    padding:0; 
    margin:0;
}
table span {
    background-color: #f00;
    border: none;
    display: block;
}
table .center {
    text-align: center;
}
table .first {
    border-left: 2px solid #888;
}

sub, sup {
    font-size: 75%;
    line-height: 0;
    position: relative;
    vertical-align: baseline;
}
sup {
    top: -0.5em;
}
sub {
    bottom: -0.25em;
}