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Measuring accessibility – Alt Tags

Tanya Goodin
Tanya Goodin
19 July 2007

The Web Content Accessibility Guidelines (WCAG) define the
criteria that allows us to measure how accessible a website is. These were
drawn up by the World Wide Web Consortium (W3C) to test the accessibility of

Measuring accessibility can be done two ways:

  • Machine testing: Determines whether or not a particular criterion has been implemented.
  • Human testing: Done by a person, success is based on whether 80% of human evaluators would agree on the implementation of the criteria.

A simple example of this is the implementation of an ‘alt’
tag on an image. The technique using a machine will only test whether the image
has an alt tag or not. It can not check that the text in the alt tag is
relevant to the image.

The human testers can come to an agreement as to whether or
not the text is appropriate to the picture. Consider, for example, an image of
a man stepping out of a car on an insurance website. Some might suggest the alt
text should be “Car Insurance”, others might say it should have a null alt
attribute as it conveys no information, whereas others may say “A man stepping
out of a car”. Who is to say, which is

Due to this inability to measure the quality of alt text,
there is no measurement of what is good alt text. The only thing we can test is
the use of an alt tag, not that it’s doing its job.

Jocelyn says – “So who decides what alt tags should say,
the client or the designer? If we all
understand the reason for alt tags we can come up with the best information for
improved accessibility”



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