Alt text is not a keyword slot.

What it is actually for

Alt text is the replacement for the image. If the picture cannot be seen, whether because someone is using a screen reader or because the file failed to load, the alt text is what stands in its place. The test is simple: read the sentence with the alt text substituted for the image and ask whether the reader has been told what they needed.

That framing answers most questions people have about it. Yes, search engines read it, and it is how an image is understood well enough to appear in image search. But it is the same text serving both, and the version written for a person is reliably the version that works for the machine.

How to write it

Describe what is in the image, specifically. "Engineer replacing a combi boiler flue" beats "boiler repair", and it will naturally contain the relevant words because the image is genuinely of that thing. That is the same principle as covering a subject properly: the vocabulary follows the substance rather than being inserted into it.

Let context do some work. Alt text does not need to repeat what the surrounding paragraph or the caption already says. It fills the gap the image leaves, not the whole page.

Keep it to a sentence. If an image genuinely needs a paragraph, such as a chart carrying data, put that in the page content where everyone benefits and keep the alt short.

Purely decorative images take an empty alt. An alt="" on a background flourish or a spacer tells a screen reader to skip it, which is correct and helpful. This is the point most audits get backwards: they flag empty alt as a fault to be filled, when for decorative images empty is the right answer and filling it adds noise.

What actually helps images in search

Good alt text is one input among several, and rarely the limiting one. A descriptive file name, the surrounding page content, a caption where a caption belongs, and image sitemaps for large libraries all contribute to understanding.

Then the unglamorous half, which usually matters more: whether the images are actually served well. Correct sizing, modern formats, lazy loading below the fold and explicit dimensions to avoid layout shift. On image-heavy sites that work moves real performance numbers, which is a bigger practical win than any amount of alt text editing. Galleries are consistently the reason a portfolio or property site is slow.

The general lesson

This one has a rare feature: the accessible answer and the SEO answer are the same answer, and there is no trade-off to manage. Write alt text for the person who cannot see the image, mark decorative images as decorative, and the search benefit arrives on its own.

It also shows how a checkable metric distorts work. "Images with alt text: 100%" is easy to measure and easy to satisfy badly, exactly like counting H1 elements. A page can pass that check while being worse for the users the attribute exists to serve.