Examples of figure captions that may be inappropriately long for alt

For discussion and possible bug-filing with regard to use of 
<figcaption> as a stand-in for alternative text when figure captions 
are over a certain length.

Background: We have had some discussions in text alternative 
sub-group meetings regarding suitability of figcaption as a stand-in 
for alternative text. The position of WAI CG several years ago was 
that <legend>, later renamed <figcaption>, was an acceptable stand-in 
for alt. This was also the conclusion in the composite HTML WG 
Co-Chairs' decision on alt conformance provided by Maciej 
http://lists.w3.org/Archives/Public/public-html/2011Apr/0451.html . 
With regard to <figcaption>, this decision stated that: "The presence 
of figcaption makes missing alt conforming."

Looking in more depth into the actual use of figure captions in 
materials published on the Web, there is great variety in the content 
and length of figure captions. It appears that although the majority 
of figure captions are reasonable stand-ins for alt, particularly for 
materials such as photos that are published informally, there are 
some types of figure captions that may not be acceptable substitutes 
for alt, such as images in some educational and scientific 
publications. In some cases, a figure caption might describe the 
processes involved in developing an image, but not provide comparable 
information to what would be in an alternative text. Those instances 
are difficult to judge, and no change is proposed to the Co-Chairs' 
decision on appropriateness of content alone. However, these 
instances often coincide with a more objective and more easily 
testable problem: some figure captions are so lengthy that they do 
not provide a comparable user experience for a screen-reader user to 
alternative text, and overly lengthy alternative text itself can be a 
problem for screen reader users. These instances could be addressed 
by not treating missing alt as conforming in the presence of 
figcaptions over a given length; or at a minimum, triggering a 
conformance warning.

I've provided examples of figure captions of varying lengths below. 
(All character counts include spaces.) Thanks to Geoff Freed for 
contributing to the examples and the discussion. The referenced 
materials were not published in HTML5, and therefore do not use 
figcaption, but they are representative of authors' practices with 
regard to captioning images in some kinds of educational and 
scientific texts. If refining alt conformance to trigger 
non-acceptance or a warning in the presence of figcaption, and the 
absence of alt, one would need to specify a threshold. The following 
examples are intended to provide some material to help that consideration.

1. http://oceanworld.tamu.edu/resources/oceanography-book/coastalerosion.htm
The first figure on this page (erosion rates along the Delmarva 
Coast) is followed by a caption that is 347 characters.

2. http://oceanworld.tamu.edu/resources/oceanography-book/coastalerosion.htm
The second figure on this page (location of Mississippi River 
channels) is followed by a caption that is 345 characters.

3. 
http://www.lightandmatter.com/html_books/genrel/ch05/ch05.html#fig:triangulation-survey
The caption is 370 characters.

4. 
http://www.lightandmatter.com/html_books/genrel/ch05/ch05.html#fig:elliptic-geom-triangle-area-proof
The caption is 377 characters.

5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2830925/figure/F4/
The caption is 534 characters.

6. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3112501/#fig1
The caption is 600 characters.

7. http://oceanworld.tamu.edu/resources/oceanography-book/co2problem.htm
The third figure on this page (average methane mixing ratios) is 
followed by a caption that is 631 characters.

8. http://oceanworld.tamu.edu/resources/oceanography-book/co2problem.htm
The second figure on this page (a chart of temperature and Co2 
concentration in the atmosphere...) is followed by a caption that is 
707 characters.

9. 
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000612#pbio-1000612-g001
The caption for this image is 892 characters overall, though split 
between a 51-character caption title, and a 841-character caption 
contents. The caption title in this case would be a suitable 
alt-substitute. This arrangement is not uncommon for captions in 
scientific publications.

10. 
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000612#pbio-1000612-g003
The caption of this image is 1689 characters overall, with a caption 
title of 114 characters that independently would be acceptable as an 
alt-substitute; and an extended caption of 1575 characters. This 
information could be provided in separate markup, but it is 
descriptive only of the experimental process, with no indication of 
results, and this particular text would not seem to be an appropriate 
substitute for alt.

Again the main question here is whether there should be a reasonable 
threshold of figcaption length above which missing alt should no 
longer be considered conforming. One suggestion was a threshold of 
250 characters; based on these examples, we may want to look at 300 
characters or higher.

- Judy


--
Judy Brewer    +1.617.258.9741    http://www.w3.org/WAI
Director, Web Accessibility Initiative (WAI), World Wide Web Consortium (W3C)
MIT/CSAIL Building 32-G526
32 Vassar Street
Cambridge, MA,  02139,  USA

Received on Monday, 1 August 2011 18:09:49 UTC