HTML and CSS Reference
10.3.8.1. The span attribute
The span attribute for the <col> tag, like for the <colgroup> tag, lets you
specify how many successive columns are affected by this <col> tag. By
default, only one is affected. For example, let's create a <colgroup> that
has five columns. We align the first and last columns to the left and
right, respectively, and center the middle three:
<col align=center span=3>
You should use the <col> tag only within <colgroup> tags that do not
themselves use the span attribute. Otherwise, the HTML 4/XHTML-com-
pliant browsers ignore the individual <col> tags and their attributes.
10.3.8.2. The other <col> attributes
The many attributes common to tables control the familiar aspects of
the column defined by the <col> tag. These attributes accept the same
values and behave exactly like the equivalent attributes for the <td> tag.
10.3.9. Using Column Groups
Column groups are easier to use than they first appear. Think of them
as a template for how to format your table columns. Their main purpose
is to create groups that can be separated by thicker rules within your
table and to streamline the process of applying formatting attributes to
all the cells in one or more columns.
Returning to our original table example, we can place a thicker rule
between the column labels and the data cells by placing the column la-
bels in one column group and the data cells in another (in HTML):