[DRAFT] Tables: concepts
All data tables need to have structural markup (coding) - elements and attributes that distinguish between header and data cells and define the relationships between them. . This tutorial shows you how to apply appropriate structural table markup. It includes the following pages:
- Header cells: Identify the topic of a row or column - header cells need to be marked up as <th> cells;
- Captions: Describe the overall topic of a table - best practice is to use the <caption> element;
- Scope of header cells: Defines the range and direction of the data cells to which a header applies - use scope values that identify the appropriate scope;
- Multi-level tables: A complex table structure where a data cell needs to reference several levels of header cells - assign each header cell an id and each data cell a headers attribute that lists all relevant header cell id values;
- Summary: Provides orientation or navigation hints in complex tables - give a brief overview as the value of the summary attribute of the <table> element or as part of the table's <caption> element.
Note: The techniques in this tutorial should be applied only when a table is being used to display data in a grid: if a table is used for layout, (which is discouraged), none of these elements and attributes should be applied.
Why is this important?
When data is presented in tabular format, the position and styling of header cells may be sufficient to let most people know that these contain key information that gives meaning to related data cell content. However, the published style is not available to people who need to use personal stylesheets, and the position alone can't help screen readers identify the cells that contain the header information. The header cells need to be explicitly identified so that correct associations can be made to data cells. Correctly marked up tables mean that:
- People using a screen reader: Can have the row and column headers read aloud as they navigate through the table. Screen readers speak one cell at a time and can only distinguish header cells from data cells if they are properly marked up.
- People using personal stylesheets: Can have header cells more prominently styled for easy recognition when there is a difference between the elements used for header and data cells.
How to make tables accessible
Accessible tables have their header cells marked up using <th> and their data cells marked up using <td>. For more complex tables directional and explicit associations may be needed, using scope, id and headers attributes so that they can be interpreted by assistive technologies. The structural coding can also be used to represent data in different ways, for example by larger or differently colored text or backgrounds, Braille, speech and symbols. Many web authoring tools and content management systems (CMS) provide functions to define header cells during table creation without having to manually edit the code. Other formats available on the Web such as ODF, Word and PDF have similar mechanisms to mark-up table structure.
Relationship to WCAG 2.0
Accessible tables are required by the following WCAG 2.0 Success Criterion: