We generally think of alt text as something that we add for users of screenreaders or text-only browsers. But what about the millions of users of search engines that index images?
The two key parameters the engine indexes are the image’s name and its alt text. (It may also make some inferences from the image’s context.) When writing alt text, think about what keywords one might use to bring that image up in a search engine, and try to include those words in a sensible, succinct phrase.
This is NOT to suggest that you fill your alt tags with SEO keyword optimizing nonsense. That rarely does the user any good. What I’m suggesting is that we consider the other means by which people search content, and the parameters by which that content is indexed.
Adding indexable meaning to your pages also makes the web more fun. For example, searching on the word “pint” in google images brings up this charming guy:
He was accompanied by this alt text:
Mr Jack Virgo enjoys a pint following a reduction in price of two pence in the 1959 budget.
Now doesn’t that make you want to write more indexable alt text? Mr. Virgo is all for it.
Further reading: TASI: A Review of Image Search Engines