Alt text is the text description that takes the place of an image for people who can't see it, and it goes in the alt attribute of the <img> tag.
The rule that decides everything else is this: the alt describes the meaning of the image on that page, not what it looks like. If the image is a button or a link, the alt states the action ("Search the site"), not the graphic ("magnifying glass icon"). If the image is pure decoration, the alt is left empty with alt="", which is not the same as leaving it out altogether.
Who actually reads alt text
A screen reader reads it aloud at the exact point where it meets the image, following the page's navigation order: the listener gets the information at the same moment a sighted user would see it. When the attribute is missing entirely, many screen readers fall back on the file name, and the listener hears IMG_4021.jpg or a path generated by the CMS.
The text also appears on screen when the image doesn't load: slow connection, moved file, images turned off in the browser. It is also one of the six most common web accessibility errors. And then there's the crawler, which doesn't see the photo but reads the string.
The three possible states of the attribute are not interchangeable, and mixing them up is the most frequent mistake.
| State of the attribute | What the screen reader does | When it is correct |
|---|---|---|
alt missing | Often reads the file name | Never |
alt="" (empty) | Skips the image silently | Decorative images that add no information |
alt="text" | Reads the text aloud | Informative images, links, buttons, logos, charts |
What Google says, in its own words
In the Google Images best practices, updated in March 2026, the official documentation singles out alt text as the most important attribute for giving Google metadata about an image. "Google uses alt text along with computer vision algorithms and the contents of the page to understand the subject matter of the image", and it treats it as anchor text when the image is used as a link.
The same page gives examples that are worth more than any list of tips. An image with no alt at all is marked as bad. alt="puppy" is only a partial improvement. alt="Dalmatian puppy playing fetch" is the recommended one. And a string of some thirty words repeating "puppy", "dog" and "dog food" is classed as keyword stuffing, and so as a breach of the spam policies.
What alt to write, image by image
The general rules hold as long as the image is a photo inside an article. Other categories need different decisions, and they are the ones audits most often find wrong.
| Type of image | What the alt should contain | Example |
|---|---|---|
| Informative photo | The information the image adds to the text | Dalmatian puppy playing fetch |
| Decorative (backgrounds, textures, dividers) | Nothing: empty attribute | alt="" |
| Image link | The link destination, not the image | Back to the Visilay home page |
| Icon button | The action triggered by the click | Print the document |
| Logo | The name in full and any text it contains | Visilay logo |
| Chart or infographic | A summary of the data, plus a long description in the text or an HTML table | Accessibility errors per home page, 2019 to 2026 |
| Image containing text | The text, word for word | The slogan text, transcribed |
Charts are worth dwelling on. A one-line alt can't convey what a sighted reader gets from a curve. The W3C techniques for success criterion 1.1.1 therefore provide for two parts: a short description in the alt and a long one elsewhere on the page. An HTML table with the same figures as the chart solves the problem better than a three-hundred-character alt, and has the side benefit of being readable by search engines too.
125 characters is not a rule
Almost every guide repeats the 125-character limit as if it were written down somewhere. It isn't: neither WCAG nor Google's documentation sets a maximum length for the alt attribute. It is a convention that grew out of some screen readers truncating long strings, and as a convention it works well: it forces you to pick the main information instead of listing everything.
Treating it as a rule, though, leads to two opposite mistakes. Cutting an infographic's description in half to stay within budget, and padding out an alt that could have been four words because "there's still room".
How many images are left without alt
The WebAIM Million 2026 runs the home pages of the top million sites in the Tranco list through the WAVE engine. The data was collected in February 2026, the sample is global and includes 25,325 home pages on .uk domains and 521,826 pages in English.
- 53.1% of home pages have at least one image with no alternative text. It is the second most common accessibility error, after low contrast.
- Home pages carry 66.6 images on average, up 13.6% in a single year. 16.2% of them have no alt, not counting intentional
alt="". - 45% of images without alt are image links: one in four clickable images produces a link no screen reader can describe.
- Among images that do have an alt, 10.8% carry repetitive or useless descriptions: "image", "chart", the file name, or the same text as the image next to it.
The UK figure is better than average. Home pages on .uk domains have 40.9 detectable errors on average against a global average of 56.1, which is 27.1% lower, and English-language pages average 46.0. Better than average still means around forty automatically detectable failures on the typical UK home page.
In the UK, alt text is also a legal matter
Alternative text is the first success criterion in WCAG: criterion 1.1.1 "Non-text Content", level A, requires all non-text content to have a text alternative that serves the same purpose. Level A is the lowest rung: if that is missing, AA is not even on the table.
In the UK two sets of rules apply, plus a third for anyone selling into the EU:
- The Public Sector Bodies (Websites and Mobile Applications) (No. 2) Accessibility Regulations 2018. Public sector websites and apps must meet WCAG 2.2 AA and publish an accessibility statement. The Government Digital Service monitors a sample of sites every year.
- The Equality Act 2010, which applies to every service provider, private businesses included: they have a duty to make reasonable adjustments for disabled people. The Act doesn't name a technical standard, but WCAG is the usual benchmark for a website.
- The European Accessibility Act (EU Directive 2019/882), in force since 28 June 2025. It doesn't apply in the UK itself, but it covers UK businesses offering services such as e-commerce and banking to consumers in the EU.
In practice one thing changes, and it's the most important: the Equality and Human Rights Commission (in Northern Ireland, the Equality Commission for Northern Ireland) enforces the public sector rules and can use investigations, unlawful act notices and court action, while a disabled user can bring a claim against a private provider under the Equality Act. Images without alt are among the first findings any automated check produces, because they are one of the few criteria a machine can verify with no room for interpretation.
How to find images without alt on your site
For a single page, the Lighthouse Accessibility audit in Chrome DevTools is enough: it flags <img> elements with no attribute. So is the WAVE extension, the same engine used for the WebAIM report. For a whole site you need a crawler: Screaming Frog has a dedicated report listing images without alt and alt text over a certain length, and it's the quickest way to see whether the problem is scattered or concentrated in a template.
It is almost always concentrated. A WordPress theme that outputs the header logo without alt produces an error on every page of the site, and it's fixed in one place. Before opening the media library and filling in four thousand fields by hand, it's worth looking at where the errors really come from: in most cases they are template sliders, logos, icons and badges, not the photos in articles. The rest of the work on images, from file size to file names, is in the guide to image optimisation for SEO.
The problem is no longer the missing alt
In the WebAIM sample, the share of images without alt fell from 18.5% in 2025 to 16.2% in 2026, while the average number of errors per page rose by 10.1%. The authors put the overall rise down to growing page complexity and to AI-assisted development practices, what the report calls "vibe coding".
This is what will change the work over the next few years. Generating alt text in bulk with a model has become trivial, and a generated alt passes any automated check: the attribute is there, it isn't empty, it isn't the file name. But a model describes what it sees in the image, not why that image is on that page, and criterion 1.1.1 asks for the second. An alt like "blue bar chart on a white background" is formally flawless and conveys nothing. The 10.8% of repetitive or useless alts that WebAIM measures today is set to grow just as the share of images without alt falls, and no tool will flag it. If you need a hand setting up an audit that looks at quality and not just at whether the attribute is present, it's one of the checks we run in our SEO projects, along with the rest of on-page SEO and accessibility checks.
Frequently asked questions
They are two different things. alt="" tells the screen reader to skip the image silently, and it's the right choice for decorative images. A missing attribute gives no instruction at all: many screen readers fall back on reading the file name, which is almost always noise.
No specification sets a limit: neither WCAG nor Google's documentation. The 125 characters you see quoted are a convention that grew out of some screen readers truncating long strings. One or two sentences work in most cases; for charts and infographics use a short alt plus a longer description in the text.
Only if it genuinely describes the image. Google advises writing useful text that fits the content of the page, and classes stuffing the attribute with keywords as spam. An accurate description already contains the right terms without forcing them in.
Google describes it as the most important attribute for providing metadata about an image and uses it, together with computer vision and the content of the page, to understand the subject. It matters most for rankings in Google Images and as anchor text when the image is a link.
Public sector bodies, under the 2018 accessibility regulations, which require WCAG 2.2 AA. Every service provider, private businesses included, has a duty under the Equality Act 2010 to make reasonable adjustments for disabled people, and WCAG is the usual benchmark for a website. UK businesses offering services such as e-commerce or banking to EU consumers also fall under the European Accessibility Act. The EHRC and, in Northern Ireland, the ECNI enforce the public sector rules.
