Alt text (alternative text) is an HTML attribute added to image elements that provides a text description of the image. Alt text serves two purposes: it describes the image to users who cannot see it (due to visual impairment using screen readers, or because the image fails to load), and it communicates image content to search engines that cannot visually interpret images. Good alt text is descriptive, accurate, and concise — typically 10 to 15 words that describe what the image shows and, where relevant, its context on the page.
Alt text is one of the most frequently poorly implemented SEO and accessibility elements on the web. Common failures include: leaving alt text blank (failing both accessibility and SEO), using identical alt text on multiple different images, stuffing keywords into alt text without describing the image ('SEO agency London best digital marketing'), and using file names as alt text ('image001.jpg'). Each of these failures represents a missed opportunity to communicate both with users and search engines.
How to write effective alt text
- Describe what is visible — 'Man in blue shirt presenting a chart to a meeting room audience' not 'business meeting'
- Be specific and accurate — alt text should describe this specific image, not a generic category of images
- Include relevant keywords naturally — if the image shows a product, include the product name; do not force unrelated keywords
- Use empty alt text for decorative images — purely decorative elements should have alt='' so screen readers skip them
- Keep it concise — under 125 characters is a common guideline; avoid lengthy paragraphs
- Avoid redundancy — do not start with 'image of' or 'picture of'; search engines know it is an image from the <img> tag
- Include text in the image — if an image contains important text (a chart label, infographic heading), include that text in the alt
Missing alt text is a lost opportunity rather than an active penalty. Pages with well-written alt text on their images gain a marginal ranking advantage over pages with blank alt attributes, because the alt text provides additional context about the page's topic. For sites where images are central to the content (ecommerce product pages, news articles with relevant photos, tutorial pages with instructional screenshots), alt text quality has a more meaningful impact on image search visibility and overall content understanding.
Alt text (alt='description') is the primary accessibility and SEO attribute — it is read by screen readers and indexed by search engines. Image title attributes (title='text') display as a tooltip when a user hovers over an image with a mouse. Title attributes are not read by most screen readers by default and are not significant SEO signals. The alt attribute is the important one for both accessibility compliance (WCAG 2.1 requirement) and search performance.