Alt text (alt attribute) – image SEO, accessibility and how to write good alternative text

Definition

Alt text (alternative text) is the descriptive text in an image's alt attribute. It is read aloud by screen readers, shown if the image does not load, and gives search engines and AI context about what the image shows. Alt text is therefore both an accessibility tool and an SEO tool.

What is alt text?

Alt text, or alternative text, is the descriptive text in an image's alt attribute in the HTML. It serves three purposes: screen readers read it aloud for people with visual impairments, it is shown as fallback text if the image cannot load, and it helps search engines and AI models understand what the image shows and why it is on the page. Alt text is therefore an accessibility tool, an SEO tool and a context signal for machines.

Missing alt text is still one of the most common errors on the web. According to WebAIM Million 2026, 53.1% of all home pages have at least one image without alt text – on average, 10.8 images per page lack a text alternative. That is a large gap, and an easy one to close for anyone who tackles it.

Alt text vs caption and title attribute – what does what?

Three things are often confused: the alt attribute, the visible image caption and the title attribute. They do completely different jobs, and only one of them is an accessibility requirement.

In short: alt text is invisible and describes the image for people who cannot see it; the caption is visible to everyone and adds context; and the title attribute is hover text that neither screen readers nor search engines can rely on. Write alt text for every meaningful image, use a caption when it adds something, and leave out the title attribute.

Four types of images – how to choose the right alt text

There is no single rule for alt text, because the right text depends on the role the image plays on the page. The W3C divides images into a few basic types, and the right choice differs markedly between them. The decision tree below sums up the logic.

• Informative images carry information that is not in the text – a product photo, a screenshot, a chart. They should have alt text that conveys the same information.
• Functional images are clickable and perform an action, such as a logo that links to the home page or an icon that opens search. Here you describe the destination or the action, not the appearance.
• Decorative images add no information – background patterns, dividers, purely atmospheric images. They should have an empty alt attribute (alt="") so that screen readers skip them.
• Complex images such as charts, graphs and infographics need more: a short alt text stating what the image shows, plus a longer description in the body text, a caption or a table next to it.

Never leave out the alt attribute entirely – not even on decorative images. The difference matters: alt="" tells the screen reader to ignore the image, while a completely missing attribute can make some screen readers read out the file name or the image URL instead.

How do you write good alt text?

Good alt text describes what the image shows and what it does, briefly and concretely, in the context of the page. It should be able to stand in for the image – someone who reads only the text should not miss anything important.

• Be specific. Describe what is actually visible and why it matters here.
• Do not write "image of" or "photo of". The screen reader already announces that it is an image.
• Describe the function of clickable images. A linked logo should say where the link goes, not what the logo looks like.
• Work keywords in naturally. Only if they belong in the description – never tacked on.
• Adapt to the context. The same image may need different alt text on different pages, depending on the point being made.

A common misconception is that alt text has to be kept under 125 characters. That limit comes from older screen reader behaviour, not from Google – some screen readers cut off long text. In practice, one concise sentence is almost always enough. Write as much as it takes to convey the point of the image, no more, and put very long descriptions in a caption or the body text instead.

Concrete examples make the difference clear:

• Informative (screenshot): poor alt="image" → good alt="Webflow Designer with an open CMS collection for blog posts".
• Functional (logo that links home): poor alt="logo" → good alt="SEOS Design – to the home page". Describe the destination, not the appearance.
• Decorative (background pattern): wrong alt="abstract purple pattern" → right alt="".
• Complex (chart): weak alt="chart" → good alt="Line chart: organic traffic grew from 200 to 1,400 visits/month in 6 months", with a supporting caption or table next to it.

Alt text and SEO – how it affects rankings

Alt text gives search engines context about images, which matters for visibility in Google Images and helps them understand the page as a whole. According to Google Search Central, Google uses alt text together with computer vision and the rest of the page's content to understand what an image shows. Google also explicitly warns against keyword stuffing in alt attributes – overstuffed alt text can be classed as spam.

So the effect is real but not magic: well-written alt text strengthens on-page SEO by clarifying the page's topic and can bring in image traffic, but it will not lift a weak page on its own. There is also an indirect benefit. A study of 10,000 websites (Semrush and AccessibilityChecker.org, 2025) found that high accessibility scores correlate with 23% more organic traffic and rankings for 27% more keywords. That is correlation, not causation – but accessibility and technical quality often go hand in hand, and alt text is one of the easiest parts to fix.

Alt text, accessibility and the law

For screen reader users, alt text is often the only way to access what an image shows. That is why it is an explicit requirement in WCAG. According to the W3C, WCAG 2.2 success criterion 1.1.1 Non-text Content (level A) requires all non-text content to have a text alternative that serves the same purpose. Decorative images meet the criterion with an empty alt (alt="") or by being hidden with CSS.

For Swedish companies, it is also a legal matter. The Swedish Act on the Accessibility of Certain Products and Services (lagen om vissa produkters och tjänsters tillgänglighet) came into force on 28 June 2025 and implements the EU accessibility directive, the European Accessibility Act (EAA). It applies to private companies offering consumer services digitally. Micro-enterprises are exempt – but only those with fewer than ten employees and a turnover or balance sheet total under €2 million. A company with eight employees and €5 million in turnover is therefore covered. The reference standard is EN 301 549, which builds on WCAG levels A and AA; alt text is a level A requirement under criterion 1.1.1. Read more about what the law means in practice in The European Accessibility Act – how the law affects your website.

Does alt text still matter when AI can "see" images?

Multimodal AI models can now interpret an image directly and describe what is in it. That raises a fair question: is alt text still needed if the AI model can see the image anyway? Yes, and for a specific reason. A model can see what the image shows, but not why that particular image is on your particular page. The alt text provides that context.

Alt text is a context signal for machines. When crawling and citing at scale, text signals are the cheapest and most reliable to read. A search engine or answer engine does not need to render and interpret every image if the description is already in the code. Google is clear that it combines alt text, computer vision and the page's content rather than relying on any one of them alone. Alt text is the part you control, and the easiest part for a machine to trust.

For AEO and being cited in AI answers such as AI Overviews, this means a well-described image becomes a clear, machine-readable fact about your page – not something a system has to guess. Read more about how AI interprets and cites content in AI Overviews and GEO.

Common myths and mistakes about alt text

• "Alt text must be under 125 characters." The limit comes from how some screen readers behave, not from Google. A concise sentence is usually enough – write as much as the point requires.
• "More keywords in the alt text give better rankings." Quite the opposite – Google warns against keyword stuffing and may treat it as spam.
• "Every image should have alt text." No – decorative images should have an empty alt (alt="") so that they are skipped.
• "AI sees the image anyway, so alt text is unnecessary." The model sees what the image shows, not why it is on the page. The alt text provides the context and is the signal machines trust most.
• "The title attribute is enough as alternative text." No – the title is only shown on hover and is not reliably read by screen readers or search engines.