Web Design in a Nutshell, 3rd Edition

Columns and Column Groups

As mentioned earlier in this chapter, the columns in a table are just implied by the number of cells in the longest row. In some instances, however, it is desirable to identify conceptual columns of data cells or groups of columns. The col (column) and colgroup (column group) elements allow authors to conceptually join a group of cells that appear in a column (or columns).

Column and column groups offer a number of conveniences. Their original intent was to speed up the display of tables in visual user agents. By specifying the width of each column, the user agent does not need to parse the contents of the entire table in order to calculate column and table. Columns and column groups are also useful for applying attributes (such as width or align) to all the cells they include. They may also be used as “hooks” for a limited number of style properties (see note). When used with the scope attribute (discussed in the upcoming accessibility section), they may also provide helpful context for screen readers and other non-visual browsing devices.

Tip

The CSS 2.1 Recommendation states that only the following four style properties may be applied to the col and colgroup elements: border, background, width, and visibility. For an in-depth explanation of why this is the case, read Ian Hickson’s blog entry, “The mystery of why only four properties apply to table columns” at http://ln.hixie.ch/?start=1070385285&count=1. See also Chapter 22 of this book for more information ...