What Is Alt Text?
Alt text (short for alternative text) is a written description added to an image using the HTML alt attribute. It tells people and machines what an image shows when the image itself cannot be seen. A screen reader announces the alt text aloud to a visually impaired user, and a browser displays it in place of an image that fails to load.
A simple example looks like this: <img src="terrier.jpg" alt="A Jack Russell terrier catching a frisbee on a beach">. The visible image and the alt text describe the same thing, so no information is lost when the picture is unavailable.
Alt text serves two audiences at once. For people who use assistive technology, it is the only way to understand visual content. For search engines such as Google, it is a primary signal for understanding what an image depicts, which feeds into both page relevance and ranking in Google Images.
Why Alt Text Matters
Accessibility is the first reason. Under the Web Content Accessibility Guidelines (WCAG), every meaningful image must have a text alternative so that screen reader users receive the same information as sighted users. Missing or unhelpful alt text is one of the most common accessibility failures on South African websites.
SEO is the second reason. Search engines cannot truly see an image, so they rely on the alt attribute, the file name, and surrounding text to understand it. Clear alt text improves your chances of ranking in image search and adds relevant context to the page as a whole. It is a small detail that compounds across every image on a large site.
There is also a practical benefit: when an image fails to load on a slow connection, the alt text appears in its place, so the page still communicates its meaning.
How to Write Good Alt Text
Describe what the image shows, concisely and accurately. Aim for a short, natural sentence rather than a list of keywords. "Woman reviewing a Google Ads dashboard on a laptop" is far more useful than "ads laptop marketing seo woman".
Keep these principles in mind: be specific but brief (roughly 125 characters or fewer, since some screen readers truncate longer text); do not begin with "image of" or "picture of" because the screen reader already announces that it is an image; include relevant text that appears inside the image; and leave the alt attribute empty (alt="") for purely decorative images so screen readers skip them. Never keyword-stuff, as it harms both accessibility and SEO.
Alt text is one element of a wider technical and on-page foundation. If you want a full review of how your images, metadata, and structure are performing, our SEO audit covers it, and our SEO services put the fixes into practice. You can also explore related terms such as meta description and schema markup.
Alt text and accessibility
Alt text, the alternative text describing an image, exists first for accessibility: screen readers read it aloud so people who cannot see an image understand what it conveys, and it displays if an image fails to load. This makes writing good alt text a matter of inclusion, not just SEO. Effective alt text describes the image's content and purpose concisely and accurately, conveying what a sighted user would gain from it, rather than stuffing keywords, which serves neither accessibility nor search. Decorative images that add no information can have empty alt text so screen readers skip them. Because search engines also use alt text to understand images, writing it well for accessibility happens to serve SEO too, but the human need comes first.
Common alt text mistakes
Several common mistakes undermine alt text. Keyword stuffing, cramming target terms into alt attributes, reads poorly to screen-reader users and can look manipulative to search engines, defeating both purposes. Missing alt text on informative images leaves screen-reader users without the content and forgoes the SEO signal. Redundant text that merely repeats a nearby caption or says "image of" adds nothing. Over-describing decorative images that carry no information clutters the experience, when empty alt text would let screen readers skip them. And vague alt text like "photo" conveys nothing useful. The fix throughout is to describe each meaningful image's content and purpose accurately and concisely, as if explaining it to someone who cannot see it, which is exactly what alt text is for.
FAQ
What is the difference between alt text and a caption?
Alt text lives in the image's alt attribute and is read by screen readers and search engines when the image cannot be seen. A caption is visible text shown beside the image for all users. The two serve different purposes and should not simply duplicate each other.
Does alt text help SEO?
Yes. Descriptive alt text helps search engines understand image content, supports ranking in Google Images, and improves overall page relevance. It should describe the image accurately rather than stuff keywords.