DocBook XML tables

We have been studying the DocBook DTD and in particular the table specifications. There are two separate and distinct formats supported, the originals CALS format and the ‘newer’ HTML format since DocBook version 4.3. CALS is an SGML standard developed by the U.S. military, and their set of table tags was one of the first to be developed that included complex features for tables.

Some confusion was encountered since one source claimed that both table formats could not be used in the same document, whilst another source claimed that the two forms cannot be mixed within the same definition, which makes more sense.

Later, because of widespread familiarity with HTML tables, DocBook added HTML table elements. Now you can use tr and td instead of row and entry in a table. They cannot be mixed within one table. Also, the content of each table cell has to be valid DocBook, so you cannot usually just cut and paste an HTML table into your DocBook document. But the DTD does permit a document to contain both CALS tables and HTML tables. The tgroup element is the distinguishing characteristic between them. A CALS table requires a tgroup, and an HTML table does not permit one.

The formats are sufficiently similar that it is worth mentioning them here. The following table neatly summarises the differences.

Purpose	HTML Element	CALS Element	Comments
Container for table elements.	`table`, `informaltable`	`table`, `informaltable`	The CALS `table` element requires a `title`, an HTML `table` element requires a `caption`. An `informaltable` accepts neither.
Table title	`caption`	`title`	Use `informaltable` for an HTML table without a caption.
Wrapper for table section	Not available	`tgroup`	HTML tables do not support subsections of a table with different column specifications. CALS requires at least one `tgroup`.
Column specifications	`col`, `colgroup`	`colspec`, `spanspec`	The CALS `spanspec` element specifies horizontal spanning (joining) of cells.
Wrapper for header rows	`thead`	`thead`
Wrapper for body rows	`tbody`	`tbody`
Wrapper for footer rows	`tfoot`	`tfoot`	In both HTML and CALS, `tfoot` must appear before `tbody`.
Row	`tr`	`row`	All open `tr` tags must have a closing tag.
Cell	`td`, `th`	`entry`	All open `td` tags must have a closing tag.
Nested table	`informaltable` inside a `td`	`entrytbl` inside a `row`	Nested HTML tables permitted since version 4.4 of the DTD.

The table format is not what I would call very pretty and we are investigating how one can use different CSS classes to make them ‘prettier’.