Alt text
The text description of an image, read by screen readers and used by search and AI systems to understand what the image shows.
In depth
What it really means
Alt text exists first for accessibility. A screen reader announces it in place of the image, so a blind user gets the same information a sighted one does. Everything else it does for you is a side effect of doing that job well.
The side effect matters more than it used to. Alt text is the only part of an image that retrieval systems can read as text, so a labeled diagram becomes findable and an unlabeled one is invisible. See multimodal search.
How it works
- The alt attribute is placed on the img tag in HTML.
- Screen readers announce it instead of the image.
- If the image fails to load, the browser displays it.
- Crawlers and AI systems read it as the text description of that image.
Pros & cons
Pros
- Accessibility compliance, which is a legal requirement in many markets.
- Makes image content retrievable to search and AI systems.
- Costs nothing beyond a sentence per image.
Cons
- Easy to do badly, and bad alt text is worse than none for screen reader users.
- No direct ranking effect, so it gets deprioritized.
- Purely decorative images need empty alt, which people forget.
Common mistakes
- Keyword stuffing. It makes the page hostile to screen reader users and does not help ranking.
- Starting with “image of” or “picture of”. The screen reader already says it is an image.
- Leaving alt off entirely, which makes the screen reader read the filename aloud.
- Writing alt text on decorative images instead of alt=””, which adds noise.
- Describing the image without conveying why it is on the page.
Best practices
- Describe what the image shows and why it matters here, in one sentence.
- Use alt=”” for purely decorative images so screen readers skip them.
- For charts and diagrams, state the finding, not just the chart type.
- Keep it under about 125 characters where you can.
- Repeat any number shown in an image in the body copy as well, so it stays retrievable.
FAQs
What is alt text?
The text description attached to an image, announced by screen readers in place of the image and read by search and AI systems as the image’s meaning.
Does alt text help SEO?
Indirectly. It is a ranking factor for image search and it makes visual content retrievable to AI systems. It is not a meaningful ranking factor for the page itself.
How long should alt text be?
One clear sentence, usually under 125 characters. Longer descriptions belong in a caption or in the surrounding body copy.
What about decorative images?
Use alt=”” with nothing inside it. That tells screen readers to skip the image rather than announcing a filename.
How do I write alt text for a chart?
State what the chart shows rather than naming the chart type. “Revenue grew 40 percent between Q1 and Q4” beats “bar chart of revenue”.
Keep reading
Related on LymLyt
Beyond LymLyt
Further reading
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