During the English Google SEO Office-Hours From November 26, 2021, Google’s John Mueller, read out a question about lazy load images and alt text:
“Alt text for lazy-loaded images: should the alt text be kind of appropriate for the image--essentially, if you have a placeholder image that is being lazy loaded?”
“So what I would watch out for here is essentially, when we render the page, we have to--or we try to--lazy load the images as well.
Because we try to load the page in a very high viewport, and usually, that triggers lazy loading, and usually, that means we can pick up the alt text and associate it with the right images.
“If you have the alt text already in place, and the placeholder image is currently there and we just see that, then I don't think that's a problem, per se.
“It's kind of like you're giving information about an image that you don't care about. But it's not that the rest of your site has a bad or a worse standing from that point of view. So I think that's kind of fine.
“The thing I would watch out for here more is that we can actually load the images that you're trying to lazy load here.
“So, in particular, we don't watch out for things like the data source attribute.
“We essentially need to see the image URL in the appropriate source attribute for the image tag, so that we can pick it up as an image.”
So, if your alt tags for the images you want to lazy load aren’t exactly accurate, that’s fine.