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James Craig, in http://lists.w3.org/Archives/Public/public-html-a11y/2013Jul/0032.html and Mathew Turvey in http://lists.w3.org/Archives/Public/public-html-a11y/2013Jul/0033.html requested statements that longdesc is inappropriate in various situations: + when an EPUB footnote is sufficient. (JC) + for Math. Use MathML instead. (JC) + for SVG graphics. Make the SVG DOM accessible instead. (JC) + for graphics of tabular data. Use an accessible table instead. (JC) + when and ordinary link is sufficient (MT)
An accessible table is usually inefficient for graphics of tabular data. Something to keep in mind is that, 1.) data visualizations and 2.) long descriptions of data visualizations have the same aim but have typically served different audiences. 1.) The aim of data visualizations (charts and graphs) has typically been to make data easier to understand to sighted people rather than simply providing raw data in a table. 2.) The aim of long descriptions of data visualizations has typically been to make the data easier to understand to people with disabilities rather than simply providing raw data in a table. Notice both aims are to *understand data*. Providing the data itself with either number one or number two is a nice touch but it is not the primary aim of either. Joe Clark talked about this some years ago in a WCAG comment [1]. He said, "To use an analogy over again, diagrams and data are like a suitcase that can be unpacked but not easily repacked. If data were understandable by themselves, we wouldn't make a chart. I can assure the Working Group that giving nondisabled people a really nice chart and disabled people a table with 10,000 or more data points does not constitute equality in any sense." WCAG revised their example after his comment to read: "A bar chart compares how many widgets were sold in June, July, and August. The short label says, 'Figure one - Sales in June, July and August.' The longer description identifies the type of chart, provides a high-level summary of the data, trends and implications comparable to those available from the chart. Where possible and practical, the actual data is provided in a table." [1] https://www.w3.org/2006/02/lc-comments-tracker/35422/understandingwcag20/789
added to the Authoring Requirements section: > Authors SHOULD NOT rely solely on longdesc where standards exist to provide direct, structured access. > Note: (informative) For example a MathML version of mathematical content, or an SVG image that uses the accessibility features of SVG, can provide better accessibility to users with appropriate technology. In such cases, it is appropriate to use longdesc as a fallback strategy, in combination with more modern techniques. Changeset: https://dvcs.w3.org/hg/html-proposals/rev/8d7c0e2d97d4 Email reply to James and Matt: http://lists.w3.org/Archives/Public/public-html-a11y/2013Dec/0045.html