What is alt text? It's the written description of an image that lives inside the alt attribute of an HTML image tag. A screen reader speaks it aloud to someone who can't see the picture, and it appears as fallback text when the image refuses to load. That accessibility job is the whole reason alt text exists. Search engines happen to read it too, which is why it gets talked about as an SEO trick, but the honest framing is the reverse: write it for humans who can't see, and the image-SEO benefit comes along for the ride.
What is alt text, in plain English?
Every image on a web page can carry a short text description. In the markup it looks like <img src="dog.jpg" alt="A golden retriever catching a frisbee">. The alt value is that "A golden retriever catching a frisbee" string. Nobody with working vision and a loaded page ever sees it. It surfaces in three situations: a screen reader announces it to a blind or low-vision user, a browser shows it when the image file is broken or slow, and a search crawler reads it to understand what the image depicts.
That first situation is the one that matters most and gets ignored most. Roughly one in four adults in the United States lives with some form of disability, and a meaningful share rely on screen readers to navigate the web. To them, an image with no alt text is either silence or a robotic recitation of the file name ("D-S-C underscore zero four nine two dot jpg"). Good alt text is the difference between a page that works for them and one that doesn't. Treat it as an accessibility requirement first and a marketing input second, and you'll never write bad alt text.
How alt text works
Alt text is a single HTML attribute, not a separate file or a tag you manage elsewhere. Most modern CMS platforms expose it as an "alt text" or "image description" field when you upload media, so you rarely touch the raw markup yourself.
The mechanics break into three audiences:
| Who reads it | When | What good looks like |
|---|---|---|
| Screen reader users | On every page load, spoken aloud | Describes the image's content and function in context |
| Browsers | When the image fails to load | Tells the reader what they're missing |
| Search crawlers | When indexing the page and image | Gives the image context for image search and on-page relevance |
There's one rule people skip: decorative images should have an empty alt attribute, written as alt="". An icon that adds nothing, a spacer, a purely ornamental flourish. Leaving the attribute off entirely is wrong (screen readers may then read the file name), but an intentionally empty alt="" tells assistive tech to skip the image cleanly. Knowing when to describe and when to stay silent is most of the craft.
For complex images (charts, diagrams, infographics) a short alt string can't carry the full meaning. The right pattern is a brief alt description plus the real data or explanation in nearby visible text or a longer caption. Don't try to cram a paragraph into the alt attribute; screen readers will read every word with no way to skim. The same precision instinct shows up in good structured data: say exactly what a thing is, in the place machines look for it.
Why alt text matters
The accessibility case is the real one, and it has teeth beyond goodwill. In the United States, web accessibility is increasingly treated as a legal obligation under the Americans with Disabilities Act, and missing alt text is one of the most common issues flagged in accessibility lawsuits and audits. The WCAG guidelines, the global standard browsers and regulators point to, list text alternatives for non-text content as a baseline requirement. This is table-stakes compliance, not a nice-to-have.
The SEO case is real but smaller, and worth being honest about. Alt text is how Google understands what an image shows, so it's the primary lever for ranking in Google Images, and it feeds a little context into the page overall. Google's own documentation tells you to write descriptive alt text for exactly this reason. If image search drives meaningful traffic for you (recipes, products, travel, anything visual), alt text is a genuine ranking input. For most informational pages it's a minor signal, part of solid on-page SEO rather than a needle-mover on its own.
There's a quieter benefit too. Clear alt text forces you to describe your images precisely, which surfaces lazy stock photos and decorative filler that add nothing. If you can't write a useful description, the image may not be earning its place. And as search shifts toward AI Overviews and multimodal models that read images alongside text, well-described visuals give those systems cleaner context to work with, which ties into how search intent gets matched to your content.
How to write alt text that works
A few rules carry almost all the value:
- Describe the content and function, in context. "A barista pouring latte art" beats "image" or "coffee." If the image is a link or button, describe where it goes, not what it looks like.
- Be specific but concise. Aim for a clear sentence, not a keyword dump and not an essay. If you find yourself past roughly fifteen words, the image is probably complex enough to need real body text instead.
- Skip "image of" and "photo of." Screen readers already announce that it's an image. Leading with "photo of a dog" wastes the listener's time.
- Use
alt=""for decorative images. Spacers, ornamental icons, and visuals that repeat adjacent text should be silent, not described. - Don't keyword-stuff. Cramming your target term into every alt attribute reads as spam to Google and as noise to a screen reader user. One natural mention where the image genuinely relates is the ceiling.
- Caption and alt are different jobs. A caption is visible to everyone and adds editorial context; alt text replaces the image for those who can't see it. They can say different things.
The most common mistake is treating alt text as an SEO field and stuffing it with keywords, which fails both audiences at once: it annoys screen reader users and trips Google's spam signals. The second most common mistake is the opposite, leaving it blank everywhere, which quietly locks out part of your audience and forfeits image search. Neither is hard to fix; both are easy to ignore. This is the kind of detail a real technical SEO and accessibility pass surfaces at scale, which is exactly what a structured SEO audit is built to catch.
The bottom line
Alt text is the text alternative for an image: written for people who can't see the picture, read by browsers when images break, and used by search engines to understand what an image shows. Get the order right. Write it for accessibility, keep it descriptive and concise, use an empty alt="" for anything decorative, and resist the urge to stuff keywords. Do that, and the image-SEO benefit takes care of itself.
It will not make or break your rankings on its own. It is one honest, low-drama input among many, and it happens to be a legal and ethical baseline you should hit regardless of what Google thinks. That combination, real obligation plus modest upside, is rare enough that there's no good argument for skipping it.
Want your images working for every visitor and every crawler? Our on-page and technical SEO work includes an image and accessibility pass: missing alt text flagged, decorative images handled correctly, and descriptions written to serve people first and search second. Email us at admin@moonsauceagency.com and we'll send back an audit of where your images are silent, broken, or invisible to search.
Keep reading: What is technical SEO? · Schema markup · Core Web Vitals · Back to the glossary
Sources: Google Search Central: image SEO best practices · W3C Web Content Accessibility Guidelines (WCAG)
