What is the limit of character to use in alt=“text” according to WCAG 2.0?

Question

What is the limit of character to use in alt="text" according to WCAG 2.0?

Answer 1

there is no limit explicitly set, but conventionally they are limited to 1024 chars. I believe that I have heard that the search engines will go no further than about 200 characters. What is the use case? There is a longdesc attribute that may be of interest...

http://www.w3.org/TR/REC-html40/struct/objects.html#adef-longdesc-IMG

Answer 2

WCAG says:

Sometimes images are worth a thousand words. Maps, Graphs and Charts are examples of images that contain information necessary to understanding the message of a page but too complex to be described in under 255 characters.

and if you really need an alternative description longer than that, you can always use longdesc.

Answer 3

Just so you know, as of this writing, HTML5 does not support longdesc attribute.

Answer 4

According to the recent WCAG2, there is no numerical limit anymore. Technique G94 says:

Providing short text alternative for non-text content that serves the same purpose and presents the same information as the non-text content

And WCAG2 is the official W3C recommendation in the accessibility field since december 2008. Other "rules" and criteria (Section 508 in US, RGAA and Accessiweb in France, etc) can be more or less restrictive than WCAG2 if they want to but it's not WCAG2 anymore.

Answer 5

The rule is 150 characters for the text associated with alt...Also, the rule is 150 characters maximum for <title> tag as well.

See here: http://www.totalvalidator.com/support/reference/wai2.html as well -- this validates against WCAG version 2.

More:

http://www6.sfgov.org/index.aspx?page=76

http://www-03.ibm.com/able/guidelines/web/webimages.html

I should also note that, even if the WCAG changes its limit, 150 characters or so is still a good general rule, since many older screen-readers won't go through more than 150 characters.