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But if I search for those words (plus others on the page to restrict the search), on some datacenters (for example 18.104.22.168 and 22.214.171.124) I am being ranked based on the alt text. I am not being ranked on others (for example 126.96.36.199 and 188.8.131.52).
Is this a glitch, or does it mark a change in Google policy?
It's something that has changed quite often in the past. I assume that Google like tweaking options often, to make it harder for people who track correlations to understand how the engine works.
It's been working for them for at least 3 years ..
the algo applied would appear to vary with site content ..
With these sites we are in a very image heavy ( page ) field ..it has always seemed to use this "Keyword stuffing" technique ..In this same "area" my site has always returned its alt texts in searches .
Look at this issue from the point of not SEO-optimized site and what's good for users.
In our SEO optimized sites, we usually put paragraph titles in <h1> and <h2> tags. But if someone optimized a page for appearance instead, the title may be made as a nice graphics. And the meaning of the text on this image is normally put into alt attribute. So the only way to get the paragraph title on such a page is to look at alt text.
And clearly Google is designed to deal with such pages, not only with SE optimized ones.
And if someone uses text browser, like lynx, or a voice browser for seeing impaired people, all images are converted to their alt texts.
So it's absolutely natural to count alt text as onpage text, even if this can be abused. But isn't it easy to detect keywords stuffing in alt and apply a penalty?
If snippets look bad with alt text, it means the page isn't properly designed - the same garbage would be perceived with Lynx or voice browser. It's webmaster's duty to design a page so that it looks logical in text version.