• Each column can be styled differently using the class selection:

td {border:solid blue 2pt}
th {border:solid red 1pt}
table {border:dotted green 3pt;  border-collapse:collapse}
colgroup.a {background-color:yellow}
colgroup.b {background-color:lime}
. . .
<colgroup class="a" span="2" />
<colgroup class="b" span="3" />
. . .

The first two columns are styled different from the last three