What is Alt text?
Alt text is a short written description of an image in its alt attribute, read aloud by screen readers, shown when an image fails to load, and used by search engines to understand what the image shows.
Also called: alt attribute, image alt tag, alternative text.
Its first job is accessibility, and that framing produces better SEO than trying to write it for a crawler. Describe what someone would need to know if they couldn't see the picture. "Plumber fitting a new thermostatic valve to a radiator" is useful to a screen reader and, incidentally, tells Google exactly what the image is.
Keyword stuffing here is an old tactic that now looks like spam to search engines and is actively unpleasant for anyone using a screen reader, who has to listen to it. One natural mention is plenty; usually the honest description contains it anyway.
Purely decorative images — a background flourish, a divider — should carry an empty alt attribute so assistive technology skips them. An empty alt is a deliberate statement; a missing one is a defect.
What to actually do about it
- Describe the image literally, in under about 125 characters.
- Use alt="" for decorative images rather than omitting the attribute.
- Don't start with "image of" — that is already announced.
- Give charts a text summary nearby; alt text alone can't carry data.
- Name files descriptively too: radiator-valve-replacement.jpg.
Koo drafts alt text for every generated and uploaded image, flags missing or duplicated alt attributes in the accessibility audit, and lets you edit any of it inline.
Related terms
Meta description
A meta description is a short summary in a page's HTML — best kept under about 155 characters — that search engines often display beneath the page title, acting as the advert for the click.
Core Web Vitals
Core Web Vitals are Google's three measured page-experience metrics: Largest Contentful Paint (loading speed), Interaction to Next Paint (responsiveness) and Cumulative Layout Shift (visual stability), all assessed on real visits, mostly mobile.