The HTML table model has evolved from studies of existing SGML tables models, the treatment of tables in common word processing packages, and a wide range of tabular layout techniques in magazines, books and other paper-based documents. The model was chosen to allow simple tables to be expressed simply with extra complexity available when needed. This makes it practical to create the markup for HTML tables with everyday text editors and reduces the learning curve for getting started. This feature has been very important to the success of HTML to date.
Increasingly, people are creating tables by converting from other document formats or by creating them directly with WYSIWYG editors. It is important that the HTML table model fit well with these authoring tools. This affects how the cells that span multiple rows or columns are represented, and how alignment and other presentation properties are associated with groups of cells.
A major consideration for the HTML table model is that the author does not control how a user will size a table, what fonts he or she will use, etc. This makes it risky to rely on column widths specified in terms of absolute pixel units. Instead, tables must be able to change sizes dynamically to match the current window size and fonts. Authors can provide guidance as to the relative widths of columns, but user agents should ensure that columns are wide enough to render the width of the largest element of the cell’s content. If the author’s specification must be overridden, relative widths of individual columns should not be changed drastically.
For large tables or slow network connections, incremental table display is important to user satisfaction. User agents should be able to begin displaying a table before all of the data has been received. The default window width for most user agents shows about 80 characters, and the graphics for many HTML pages are designed with these defaults in mind. By specifying the number of columns, and including provision for control of table width and the widths of different columns, authors can give hints to user agents that allow the incremental display of table contents.
For incremental display, the browser needs the number of columns and their widths. The default width of the table is the current window size (width="100%"). This can be altered by setting the width attribute of the TABLE element. By default, all columns have the same width, but you can specify column widths with one or more COL elements before the table data starts.
The remaining issue is the number of columns. Some people have suggested waiting until the first row of the table has been received, but this could take a long time if the cells have a lot of content. On the whole it makes more sense, when incremental display is desired, to get authors to explicitly specify the number of columns in the TABLE element.
Authors still need a way of telling user agents whether to use incremental display or to size the table automatically to fit the cell contents. In the two pass auto-sizing mode, the number of columns is determined by the first pass. In the incremental mode, the number of columns must be stated up front (with COL or COLGROUP elements).
HTML distinguishes structural markup such as paragraphs and quotations from rendering idioms such as margins, fonts, colors, etc. How does this distinction affect tables? From the purist’s point of view, the alignment of text within table cells and the borders between cells is a rendering issue, not one of structure. In practice, though, it is useful to group these with the structural information, as these features are highly portable from one application to the next. The HTML table model leaves most rendering information to associated style sheets. The model presented in this specification is designed to take advantage of such style sheets but not to require them.
Current desktop publishing packages provide very rich control over the rendering of tables, and it would be impractical to reproduce this in HTML, without making HTML into a bulky rich text format like RTF or MIF. This specification does, however, offer authors the ability to choose from a set of commonly used classes of border styles. The frame attribute controls the appearance of the border frame around the table while the rules attribute determines the choice of rulings within the table. A finer level of control will be supported via rendering annotations. The style attribute can be used for specifying rendering information for individual elements. Further rendering information can be given with the STYLE element in the document head or via linked style sheets.
During the development of this specification, a number of avenues were investigated for specifying the ruling patterns for tables. One issue concerns the kinds of statements that can be made. Including support for edge subtraction as well as edge addition leads to relatively complex algorithms. For instance, work on allowing the full set of table elements to include the frame and rules attributes led to an algorithm involving some 24 steps to determine whether a particular edge of a cell should be ruled or not. Even this additional complexity doesn’t provide enough rendering control to meet the full range of needs for tables. The current specification deliberately sticks to a simple intuitive model, sufficient for most purposes. Further experimental work is needed before a more complex approach is standardized.
This specification provides a superset of the simpler model presented in earlier work on HTML+. Tables are considered as being formed from an optional caption together with a sequence of rows, which in turn consist of a sequence of table cells. The model further differentiates header and data cells, and allows cells to span multiple rows and columns.
Following the CALS table model (see [CALS]), this specification allows table rows to be grouped into head and body and foot sections. This simplifies the representation of rendering information and can be used to repeat table head and foot rows when breaking tables across page boundaries, or to provide fixed headers above a scrollable body panel. In the markup, the foot section is placed before the body sections. This is an optimization shared with CALS for dealing with very long tables. It allows the foot to be rendered without having to wait for the entire table to be processed.
For the visually impaired, HTML offers the hope of setting to rights the damage caused by the adoption of windows based graphical user interfaces. The HTML table model includes attributes for labeling each cell, to support high quality text to speech conversion. The same attributes can also be used to support automated import and export of table data to databases or spreadsheets.