- From: Chris Ridpath <chris.ridpath@utoronto.ca>
- Date: Mon, 10 May 2004 14:42:44 -0400
- To: "WAI WCAG List" <w3c-wai-gl@w3.org>
We've been looking at using the "TH" rule to differentiate different HTML table types. The proposal is that all data tables must have TH elements while all layout tables cannot have TH elements. This would make it easy for assistive technologies to determine the table type and render the information more accurately to the user. I took an action to look at how this may impact tables as they are currently used on the web. As examples of real word tables, I used the Yalin Wang database of tables [1]. Of the over 14000 tables in the database, I looked at a random sample of about 2000 tables and graded them as layout or data. In most cases it was easy to decide the table type but I discovered several tables that didn't fit into either category. (If you're interested in grading these tables yourself, I've written a small program to assist the process. Please contact me for more info.) In some cases, authors combined both data and layout into one table [2]. This is very confusing to the viewer and we should discourage this practice. Use a table for one purpose only. The majority of tables in use on the web are used for layout. In my sample 89% of the tables were layout, only 11% were data. We're asking authors to use CSS instead of TABLE elements but the reality is that layout tables are in wide use. Only a very small number (< 1% in my sample) of layout tables use TH elements. This is an improper use of TH when other markup such as a heading or STRONG should be used [4]. Not a big problem but we should continue to discourage authors from doing this. Most data tables are missing TH elements. In my sample a whopping 87% of the data tables were without TH elements. Most authors leave off the headings of their data tables or mark one of the TD elements incorrectly as B or STRONG [3]. We need to more strongly encourage authors to put TH elements into their data tables. Exceptions to the TH rule: Tables are often used to layout form controls. This is a kind of layout table but also acts like a data table because the cells can't be moved without affecting their meaning [5]. In this type of table the TH elements are not as necessary because each control has an associated label. I suggest that tables used to layout form controls can have an optional row/column of TH elements. Tables are sometimes used to display "tables of contents" [6]. These are really data tables but I don't think they require TH elements. A "table of contents" is generally understood to include a title (e.g. chapter name) associated with a location (e.g. page number). When the viewer encounters a number within a table of contents they don't need to be reminded that this is the page number, they already know that because of the table's purpose. I suggest that tables used for "table of contents" purposes may have optional TH elements. Discussion: All tables with only one row are layout tables. The scope attribute must also be used with TH elements. In my testing with Jaws 4.5 I noticed that the TH elements were not read with the data cells unless the TH elements also had the scope attribute. I've heard that other screen readers behave the same way. Assistive technologies should be able to use the TH element alone but it appears that since the TH rule is not widely used they have taken to use the scope attribute. Cheers, Chris [1] http://www.math.harvard.edu/~ylwang/doc/webtablegrnd.zip [2] http://tile-cridpath.atrc.utoronto.ca/acheck/servlet/TDShowTable?file=www.airtalk.com.schedule.shtml&tabid=06468 [3] http://tile-cridpath.atrc.utoronto.ca/acheck/servlet/TDShowTable?file=www.washingtonpost.com.wp-srv.sports.longterm.olympics1998.sport.figskate.articles.figskate.htm&tabid=14018 [4] http://tile-cridpath.atrc.utoronto.ca/acheck/servlet/TDShowTable?file=industry.ebi.ac.uk.w2h.Doc.help.mangpage.html&tabid=05329 [5] http://tile-cridpath.atrc.utoronto.ca/acheck/servlet/TDShowTable?file=www.blackwellpublishers.co.uk.static.figure.htm&tabid=06819 [6] http://tile-cridpath.atrc.utoronto.ca/acheck/servlet/TDShowTable?file=www.nap.edu.books.0309070775.html.index.html&tabid=09837
Received on Monday, 10 May 2004 14:43:29 UTC