Alt text is a short written description of an image, placed in the image’s HTML as the alt attribute, that tells people and machines what the image shows. Screen readers read it aloud to blind and partially sighted users, browsers display it when an image fails to load, and search engines use it to understand the image and the page around it.
How alt text works
In the code, it looks like this: <img src="boiler-service.webp" alt="Engineer checking the pressure gauge on a combination boiler">. Most content management systems, including WordPress, give you an “Alternative text” field in the media library or the image block, so you rarely need to touch the code.
A screen reader announces the image and reads the description in the flow of the page. If the attribute is missing, many screen readers read out the file name instead, which might be “IMG_4421.jpg”. If the attribute is present but empty (alt=""), the screen reader skips the image, which is the correct treatment for decoration such as a background pattern or a divider.
Google uses alt text alongside the file name, caption and surrounding text to work out what an image shows, which affects whether it appears in image search and Google Lens results. It is one of several signals covered under image SEO.
Why it matters
For a UK business, the first reason is people. The Equality Act 2010 requires service providers to make reasonable adjustments for disabled people, and an inaccessible website can fall within that. The accepted measure of an accessible site is WCAG at level AA, which requires a text alternative for every meaningful image. Public sector bodies have a specific legal duty to meet it.
The second reason is search. Product photos, project galleries and diagrams with clear alt text can bring visitors from image search, and the descriptions reinforce what the page is about. For an online shop, a product image described as “Navy wool peacoat, double-breasted, shown on a model” is far more useful than “product1”.
Common mistakes
- Leaving the alt attribute out, so screen readers fall back to the file name.
- Writing “image of” or “photo of”. Screen readers already announce that it is an image.
- Stuffing keywords: “SEO consultant London best SEO London SEO expert”. This helps nobody and reads as spam.
- Copying the page title or the same phrase onto every image.
- Describing decorative images at length, which slows down screen reader users.
- Using images of text, such as an offer banner, with alt text that does not repeat the words.
- Writing alt text for a linked image that describes the picture rather than where the link goes.
How to act on it
Run a crawl or a Lighthouse accessibility check to list images with missing alt attributes, then work through the important pages first: home, services, products and contact. For each image, ask what a person who cannot see it needs to know in this context. Usually one short sentence does the job. If the image is a chart, summarise the point it makes in the alt text or in the surrounding copy.
Mark decorative images with an empty alt attribute rather than leaving it missing. Make writing alt text part of uploading any new image, so the backlog does not return. Reviewing images, headings and page copy together is part of the on-page SEO work I do for UK sites.