Google's John Mueller clarified in yesterday's webmaster hangout video that while Google does look and read the alternative image attribute (alt tag) for use on web search, it plays a small role there. It is more heavily used by Google in image search and that is the primary SEO reason to use alt image text, not for web search but for image search.
John said this at the 9:40 minute mark into the video:
Here is the transcript:
Does the alt image tag play a role in rankings for non image search? Should we be putting descriptive comments on what the pictures are to help with general search results.So the text in the alt attribute for image tags is seen as a part of the text on the page. So that's something where it does help us a little bit to understand that page better. But for most cases it'll be that we already have this text on the page somewhere anyway. So just by also having it in an alt attribute for an image tag wouldn't change anything.
It does play a big role for image search though. So if you have images that you want to have appear in Google images then make sure that you have a clean alt text there. Also make sure that you watch out for the other aspects that relate to image search as well.
So for web search - it is used a little bit.
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