Alt Text Writing

Alt text writing

Alt text is what a screen reader speaks in place of an image, and what shows when an image fails to load. Good alt text conveys the image's purpose in context, not a literal description; the most common mistakes are missing alt, useless alt ("image"), and describing decorative images that should be silent.

Method

1. Convey purpose in context, not literal description. The same image needs different alt text depending on why it is there: a photo of a person might be "Dr. Jane Smith" in a bio, "Team member smiling" in a culture page, or nothing if purely decorative. Ask "what does this image communicate here, and what would a sighted user take from it?": that meaning is the alt text, not an inventory of pixels.
2. Mark decorative images empty. Images that add no information (backgrounds, dividers, decorative flourishes, an icon next to text that already says the same thing) get alt="" (empty, not missing): so the screen reader skips them entirely. Decorative images with descriptive alt, or with no alt attribute at all (screen readers then read the filename: "IMG_4021.jpg"), are noise that clutters the experience.
3. Describe functional images by their action. An image that is a link or button (a magnifying-glass search icon, a logo linking home) gets alt describing the function, not the appearance: "Search", "Home", not "magnifying glass icon". The user needs to know what activating it does (see aria-usage's function-not-appearance rule for icon buttons), because that is the image's purpose.
4. Keep it concise; put length where it belongs. Alt text is typically a phrase or short sentence; a screen reader reads it in full with no formatting, so a paragraph is tiresome. For images needing long description (a complex chart, a detailed diagram), provide a short alt plus a longer description in the surrounding text or a linked/adjacent description (see the complex-figure handling below).
5. Handle complex images with text alternatives. Charts, graphs, infographics, and diagrams carry data that a short alt cannot: provide the information in accessible text (a data table for a chart, a text summary of a diagram: see the data-behind-the-visual in scientific-visualization) so the content, not just "bar chart", is available. The image is a convenience for sighted users; the text is the accessible source of the same information.
6. Do not start with "image of" and do not stuff keywords. Screen readers already announce it as an image, so "image of a cat" is redundant ("a cat" suffices); and alt text is for users, not SEO keyword stuffing (which degrades the experience and search both: see technical-seo's honest-markup rule). Write for the person listening.

Boundaries

• Alt text quality depends on context and authorial intent; it cannot be fully automated (AI-generated alt describes content but often misses purpose-in-context), though it can draft a starting point a human refines. The purpose-in-context judgment is human.
• Alt text serves images; other media (video, audio) need captions, transcripts, and audio descriptions, which are separate techniques for separate content types.
• Empty alt (decorative) versus descriptive alt versus functional alt are three distinct decisions per image; the frequent failure is applying one rule to all images rather than deciding each image's role.