I came here because currently the WordPress table block doesn't support the colspan parameter and i thought i will replace it using CSS. This was my solution, assuming that the columns are the same width:
table {_x000D_
width: 100%;_x000D_
}_x000D_
_x000D_
table td {_x000D_
width: 50%;_x000D_
background: #dbdbdb;_x000D_
text-align: center;_x000D_
}_x000D_
_x000D_
table tr:nth-child(2n+1) {_x000D_
position:relative;_x000D_
display:block;_x000D_
height:20px;_x000D_
background:green;_x000D_
}_x000D_
_x000D_
table tr:nth-child(2n+1) td {_x000D_
position:absolute;_x000D_
left:0;_x000D_
right:-100%;_x000D_
width: auto;_x000D_
top:0;_x000D_
bottom:0;_x000D_
background:red;_x000D_
text-align:center;_x000D_
}
_x000D_
<table>_x000D_
<tr>_x000D_
<td>row</td>_x000D_
</tr>_x000D_
<tr>_x000D_
<td>cell</td>_x000D_
<td>cell</td>_x000D_
</tr>_x000D_
<tr>_x000D_
<td>row</td>_x000D_
</tr>_x000D_
<tr>_x000D_
<td>cell</td>_x000D_
<td>cell</td>_x000D_
</tr>_x000D_
</table>
_x000D_