Alt Tag: SEO and Image Accessibility
6 min
The alt tag is an HTML attribute that describes the content of an image for search engines and screen readers. In SEO, it helps Google understand and index images. In accessibility, it is mandatory for visually impaired users. An effective alt tag is descriptive, contextual, and naturally integrates the keyword if relevant.
Invisible to most visitors, the alt tag is nonetheless read by Google, screen readers, and all contexts where an image cannot be displayed. Writing it carefully is both good SEO practice and an accessibility requirement.
The Role of the Alt Tag in SEO
Google cannot 'see' an image the way a human can. It uses the alt tag, the file name, and the surrounding text to understand what the image represents and index it in Google Images.
A well-written alt tag can generate traffic from image search, particularly for e-commerce sites, portfolios, and visual content blogs.
The alt tag also contributes to the page's semantic context: a well-described image reinforces the thematic coherence perceived by Google.
How to Write an Effective Alt Tag
Describe what the image actually shows, as if explaining it to someone who cannot see it. Be precise without being exhaustive: 5 to 15 words are generally sufficient.
Integrate the page's main keyword if and only if it is naturally relevant to the description of the image. Do not force a keyword into an image that has nothing to do with it.
For purely decorative images (separators, backgrounds), use an empty alt (alt='') : screen readers will skip the image without announcing it, avoiding an unnecessary interruption.
- Describe the main subject of the image (who, what, where if relevant).
- Include the keyword if the description calls for it naturally.
- Avoid unnecessary prefixes: no 'Image of' or 'Photo of'.
- Keep under 125 characters for screen readers.
- Decorative images: alt='' (attribute present but empty).
E-commerce sites that fill in descriptive alt tags on all their product images record on average 15 to 25% additional organic traffic coming from Google Images.
Industry studies 2025-2026 on e-commerce image SEO
Common Mistakes and How to Avoid Them
Keyword stuffing in alt tags is the most common and counterproductive mistake. An alt like 'SEO Paris SEO agency natural referencing Paris cheap' helps neither Google nor users.
The complete absence of an alt tag (missing attribute, not just empty) prevents Google from evaluating the image and generates an accessibility warning. Systematize checking via a regular technical audit.
Identical alt tags on multiple images on the same page (all labeled 'product-photo.jpg' or 'illustration') are treated as duplicate content by Google.
FAQ
Is the alt tag legally required?
In many countries, yes. The WCAG guidelines at the international level require that all informative images have a text alternative. For decorative images, the alt attribute must be present but empty.
Does the alt tag impact Core Web Vitals?
Indirectly. It does not affect speed metrics, but the width and height attributes that must accompany each image impact CLS. These attributes and the alt tag are part of the same set of best practices to check simultaneously.
Should alt tags be written in the language of the page?
Yes, in the main language of the page. For an English-language site, write in English. Google understands multilingual alt tags, but consistency with the language of the surrounding content is preferable for semantic relevance.