What is Alt text?

Alt text is the value of the alt attribute on an <img> element. It is a text replacement for the image: screen readers announce it, browsers display it when the file fails to load, and search engines read it to understand what the picture shows and how it relates to the page.

The attribute exists first for accessibility. The MDN reference and the W3C alt decision tree both frame it the same way: write what a person would need to know if they couldn’t see the image. Good SEO follows naturally from that.

Why it matters for search

Google’s image SEO best practices say it uses alt text, together with the surrounding content and the file name, to understand image subject matter. That affects whether your images appear in Google Images and in visual results on the main results page. Alt text on a linked image also acts like anchor text for the link.

For AI search, alt text is one of the few places where the meaning of an image is stated in plain HTML. Crawlers that read server HTML without analyzing pixels can only “see” a chart, diagram or product photo through its alt text and caption.

Good, bad and empty

<!-- Too vague -->
<img src="IMG_4821.jpg" alt="image">

<!-- Keyword stuffing -->
<img src="boots.jpg" alt="hiking boots cheap hiking boots best hiking boots sale">

<!-- Useful -->
<img src="trail-boot-sole.jpg"
     alt="Close-up of a hiking boot’s rubber sole with deep zigzag lugs"
     width="800" height="533" loading="lazy">

<!-- Decorative: empty alt so screen readers skip it -->
<img src="divider-wave.svg" alt="">

The empty alt="" is deliberate and correct for purely decorative images. Leaving the attribute out entirely is different: screen readers may fall back to reading the file name aloud.

Writing alt text that works

• Describe what the image shows in this context. The same photo of a laptop might be “Yes SEO report showing a score of 82” on a tool page and “Person working at a café table” on a blog post about remote work.
• Keep it concise — usually one sentence. Put long explanations of complex charts in the surrounding text or a caption.
• Skip “image of” or “picture of”; assistive technology already announces it as an image.
• Use your target phrase only if it honestly describes the picture.
• For images of text, such as a logo or a quote card, repeat the text itself.

Common mistakes

The usual problems are missing attributes on product galleries generated by a CMS, the same alt text copied onto every image in a template, file names used as alt text (“DSC_0042”), and icons inside buttons or links with no text alternative at all, which leaves the link with an empty accessible name.

How to check

The Image Alt Text Checker lists every image on a page with its alt text, file size, format, dimensions and lazy loading status, and flags missing alt attributes, oversized files and layout-shift risks. Alt text is also part of a broader on-page SEO review, so fix it at the same time as titles and headings.

Frequently asked questions