Web designers aren’t generally malicious
and they usually don’t actively avoid assigning alternative text or purposely
assign incorrect alternative text.
Assigning
alternative text is often a subjective process and many web designers simply
don’t know what alternative text to provide.
Because the majority of web designers are not blind, they don’t see the
immediate reward for providing alternative text.
Complicating matters, some images need alternative text and some images don’t.
If an image conveys information to a sighted
user then it should be assigned alternative text that conveys that information
to a blind user.
If an image does
something when you click on it, then it needs alternative text.
Conversely, if it’s purpose is inherently visual – if it’s used for decorative
or layout purposes – then it should be assigned a zero-length alternative
text.
Simply omitting the alt attribute
for such images is not good because the screen reader does not know the
significance of the image in this case and will announce the image.
A common example of this are images that are
used for spacing on web pages. Often these images are used 10s ot 100s of
times on a single web page. Forgetting to assign zero-length alternative text
may cause the user to hear “spacer.gif” repeated multiple times on that
page.
Using our subjective idea of what significance is as a guide, we presented an automatic
test that correctly determines significance in nearly all cases.