In PrimeFaces 3.0, that style get applied on the generated inner <div>
of the table cell, not on the <td>
as you (and I) would expect. The following example should work out for you:
<p:dataTable styleClass="myTable">
with
.myTable td:nth-child(1) {
width: 20px;
}
In PrimeFaces 3.5 and above, it should work exactly the way you coded and expected.