Alt text

Long definition

Alt text describes what an image conveys to a user who cannot see it. The mechanism is the HTML attribute <img src="..." alt="...">. Screen readers read the alt value out loud; if the image fails to load, browsers display the alt value in its place; crawlers index it.

For SEO, alt text plays two distinct roles. When an image is wrapped in a link (<a><img></a>), Google explicitly treats the alt as the anchor text for that link — the same relevance signal that text anchors carry. When the image stands alone, alt is a content signal that helps Google Images rank the asset and helps AI Overviews and LLM scrapers caption what the image shows. Pages with descriptive alt text on hero images consistently win more image-pack placements than pages with empty or filename-derived alt.

Decorative images take alt="" (empty string, not omitted). The empty alt tells assistive tech to skip the image entirely. Background patterns, dividers, and pure decoration belong here. Omitting the attribute makes screen readers fall back to the filename — usually unreadable — so always include alt, even if empty.

Length and content follow W3C WAI guidance: describe the function the image serves in context. A product photo on a PDP: "Black leather Chelsea boot, side profile". A chart: describe the data point the chart is making, not "bar chart". A logo in a header link to home: the company name, since the link target is home and the logo's function is "go home".

Avoid keyword stuffing — Google has called this out as a manipulation pattern, and screen-reader users find stuffed alt text actively painful to listen to. One descriptive phrase per image, no repetition of the same keyword across every image on the page.

Common misconceptions

• "Alt text is only for accessibility." It's load-bearing for both accessibility and SEO. Google Images, image-pack rankings, and image-link anchor signals all read alt.
• "Skip alt on decorative images." Wrong — use alt="". Omitting the attribute makes screen readers announce the filename. The empty value is the explicit "skip me" instruction.
• "Filenames replace alt text." Google reads filenames as a weaker secondary signal. chelsea-boot-black.jpg helps, but it doesn't replace alt="Black leather Chelsea boot, side profile".
• "Long alt text is better." Beyond ~125 characters, screen readers may truncate or split awkwardly, and Google has no documented preference for length. Aim for the shortest accurate description.