
There is a lot of confusion about the use of “Alt” attribute for images. A number of SEO guides suggest inserting keywords into ALT tags. The truth is that the “Alt” attribute started getting misused a long time ago.
Here are two different “official” recommendations for using the “Alt” attributes.
How to specify alternate text (W3C recommendation)
“Several non-textual elements (IMG, AREA, APPLET, and INPUT) let authors specify alternate text to serve as content when the element cannot be rendered normally. Specifying alternate text assists users without graphic display terminals, users whose browsers don’t support forms, visually impaired users, those who use speech synthesizers, those who have configured their graphical user agents not to display images, etc.”
W3C recommends caution while using the Alt attribute because using meaningless (keyword stuffed or irrelevant) alternate text will slow down user agents that must convert text to speech or braille output.
W3C – on “Alt” Attribute.
Good practices for images (Google recommendation)
“Supply alt text when using images as links- If you do decide to use an image as a link, filling out its alt text helps Google understand more about the page you’re linking to. Imagine that you’re writing anchor text for a text link. “
The recommendation from Google to use Alt attribute for links is sort of counter to the W3C recommendations. The correct attribute to use for links should have been the “title” attribute.
Google SEO Starter Guide.
Supply alt text when using images as links- If you do decide to use an image as a link,
filling out its alt text helps Google understand more about the page you’re linking to. Imagine
that you’re writing anchor text for a text link.
Avoid:
writing excessively long alt text that would be considered spammy
using only image links for your site’s navigation