Alt text is the short written description attached to an image on a web page. It exists mainly for people who cannot see the image, including visitors who use screen readers, and it also appears when an image fails to load. Good alt text answers a simple question: what would a visitor miss if this image were removed? If the answer is nothing important, the image may not need a description at all. If the answer is a key detail, a chart, or a step in a process, the alt text should carry that information in plain words.
Describe the point, not every pixel
Start with the purpose of the image. A photograph of a designer sketching a layout might need only a short phrase, such as a designer sketching a website layout on paper. A chart needs its main finding, not a list of every bar and label. A decorative divider or a purely visual background image, on the other hand, is best left with empty alt text, so screen reader users are not interrupted by a description of something that adds no meaning. Length matters too. Most useful descriptions fit in one sentence, and very long alt text becomes hard to follow when read aloud.
Avoid starting with phrases such as image of or picture of, because the screen reader already announces that an image is present. Also avoid stuffing keywords into alt text. A description written for search engines rather than people usually reads awkwardly and helps no one. The same honest approach runs through on-page SEO basics every designer should know: text that serves readers first tends to serve search engines as well, without tricks.
Fit alt text into your normal workflow
The easiest way to write good alt text is to decide it when you choose the image, not as an afterthought before publishing. Ask what the image adds to that section, write one sentence, and move on. If you cannot explain what the image adds, consider whether a different image, or no image, would serve the page better. For images that carry essential information, such as a diagram or a price table shown as a picture, put the information in the page text as well, so it is available to everyone and easy to update.
Finally, review alt text when you review a page. Images change, captions change, and a description that once matched may no longer fit. A quick read-through catches those mismatches before visitors do. This attention to images also supports performance, which is why improving Core Web Vitals through smarter design choices pairs well with this guide: fast, well-described images make a page better for every visitor, not just for search engines. Write for the person who cannot see the image, keep it short, and your alt text will do its job.