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 ...
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