What is alt text, and how does it help SEO?

What alt text is

Alt text is the value of the alt attribute on an img element. It exists so that people who cannot see an image still get what it communicates. A screen reader announces it, a browser displays it when the image is broken, and search engines read it as a description of the image.

<img src="/team-photo.jpg" alt="The five-person support team at the Brno office">

How it helps SEO

Google says it uses alt text, together with the surrounding page content and the file name, to understand the subject of an image. That matters most for Google Images and for image results inside normal search. Alt text on a linked image also works like anchor text for the link.

What it does not do: stuffing keywords into alt attributes does not lift a page's ranking, and Google lists keyword stuffing in alt text as something to avoid. Write for the person who cannot see the image. That is also the version search engines understand best.

How to write good alt text

• Describe what the image contributes in this context, not every visible object. The same photo needs different text in a news story and in a product gallery.
• Keep it to a sentence. There is no fixed character limit, but a paragraph inside an alt attribute is the wrong place for detail.
• Do not start with image of or picture of. Screen readers already announce that it is an image.
• For a linked image, describe the destination or action, such as Home or Download the price list.
• For a chart, state its main point in the alt text and put the data in the page as text or a table.
• For text inside an image, put the same text in the alt attribute, and prefer real text where you can.

An empty alt attribute is correct for decoration, such as a divider or a background flourish. A missing alt attribute is not the same thing: screen readers may read out the file name instead.

Common mistakes

• File names as alt text, such as IMG_2041.jpg or hero-final-v3.
• The same alt text on every image, often the page title, generated by a CMS default.
• Alt text that repeats the caption word for word right next to it.
• Meaningful images placed as CSS backgrounds, which cannot carry alt text at all.
• AI-generated descriptions accepted without review. They describe pixels, not purpose.

Where builders and frameworks go wrong

AI app builders and component libraries often render images from data with an alt value bound to a field that is empty, or omit the attribute entirely. In React, an img without alt also triggers a lint warning that is easy to ignore. Check the rendered HTML, not just the component code.

What a scan can and cannot tell you

WebAuditOS counts img elements without an alt attribute on each page. No missing attributes passes, up to thirty percent missing warns, and more fails. It also inspects image-URL format hints and checks responsive markup when at least three eligible images are present; it does not fetch the images to verify their formats.

An empty alt counts as present, because it is correct for decoration. The scan cannot judge whether your wording is useful, whether an empty alt hides important information, or whether a CSS background carries meaning. That takes a person reading the page.

A pass means every image has the attribute. It does not mean every image is described well.

Short answers

How long should alt text be? One short sentence is usually enough. Longer content belongs in the page.

Does alt text help ranking? It helps Google understand images and image search. It does not boost a page by itself.

Should every image have alt text? Every img should have the attribute. Decorative images get an empty one.

Can I use AI to write alt text? As a draft, yes. Review each one for what the image means on that page.