How Alt Text Impacts SEO and Accessibility

Introduction

Images play a critical role in modern web content. They communicate ideas faster than text, improve engagement, and support visual storytelling. But images alone cannot fully convey their meaning to search engines or assistive technologies — and that gap is precisely what image alt text exists to close.

Alt text is the alt attribute on an HTML image element. It provides a textual description that search engines use to understand what an image depicts and that screen readers use to describe images aloud to users with visual impairments. Properly written alt text improves both SEO image performance and web accessibility, making websites easier to discover and more inclusive for everyone.

This guide covers how alt text affects search rankings, what accessible alt text actually requires, how to write it correctly across different image types, and the most common mistakes that quietly damage both SEO and user experience. Developers who want the broader image optimisation picture alongside alt text should also work through the SEO image optimisation checklist for developers, which covers alt text as one of ten interconnected techniques.

What Is Image Alt Text?

Alt text is a short description added to the alt attribute of an <img> element:

<img src="optimized-image.webp" alt="developer optimizing website images for faster page speed">

It serves three distinct purposes:

For search engines: alt text provides the primary textual signal for understanding what an image shows, how it relates to surrounding content, and which search queries it should appear for in image search.

For screen readers: alt text is read aloud to visually impaired users in place of the image itself. A user who cannot see the image depends entirely on the alt text to understand what visual information is being communicated.

As fallback content: when an image fails to load — due to a slow connection, a broken URL, or a user who has images disabled — the alt text displays in place of the image, ensuring the page remains understandable.

Alt text is therefore a critical bridge between visual content and the text-based systems — search engines and assistive technologies alike — that interpret it.

Why Alt Text Matters for SEO

Search engines cannot see images the way humans do. Even with advances in AI-based visual recognition, crawlers still rely heavily on textual context — including alt text, surrounding copy, file names, and page structure — to understand what an image depicts and how it relates to a query.

Well-written alt text helps search engines understand the image content, connect the image with relevant keywords, index images accurately in image search results, and strengthen the topical relevance of the page as a whole.

The difference in practice is significant. Consider an article about image compression that includes a screenshot of a compression workflow. If the alt text reads:

image1.png

Search engines gain nothing useful. The image is invisible to the index from a semantic standpoint.

If the alt text reads:

developer compressing website images using a browser-based compression tool

The image now reinforces the page's topic, supports the surrounding keyword context, and has a chance of appearing in image search results for relevant queries. The same image, the same page — the only difference is the alt attribute.

Alt Text and Image Search Visibility

Image search is a meaningful traffic source that many developers overlook. When users search visually descriptive queries — "website performance optimisation diagram", "image compression workflow", "responsive image layout example" — search engines surface images whose alt text, filenames, and surrounding content match the query.

Properly optimised alt text is the single most controllable signal for image search visibility. A well-described image on a well-optimised page can generate consistent organic traffic from image search, entirely separate from its page-level rankings.

Understanding how images appear in AI search results extends this further — in AI-generated answer experiences, images with strong contextual signals including alt text are more likely to be surfaced inline within answers, not just in the image tab.

The Accessibility Impact of Alt Text

Alt text is a foundational accessibility requirement, not an optional enhancement. Screen readers — software used by people with visual impairments to navigate the web — read the alt attribute aloud when they encounter an image. Without it, the user hears nothing, or the screen reader falls back to reading the filename (e.g. "IMG underscore 20394 dot png"), which communicates nothing useful.

The accessibility impact extends beyond screen reader users. People with cognitive disabilities benefit from clear, descriptive image descriptions. Users on slow connections who disable image loading still need to understand what images are meant to show. Alt text serves all of these cases simultaneously.

In many jurisdictions, web accessibility is also a legal obligation. The Web Content Accessibility Guidelines (WCAG) require meaningful images to have descriptive alt text as a Level A requirement — the baseline level of compliance. Websites that fail this requirement face both exclusion claims and legal exposure in regions with digital accessibility legislation.

When to Use Alt Text — and When Not To

The most common misconception about alt text is that every image needs a description. It doesn't. The correct approach depends on the image's purpose.

Informational Images

Images that convey meaningful content — diagrams, screenshots, photographs that illustrate a concept — need descriptive alt text that communicates what the image shows.

alt="diagram showing responsive image srcset delivering different sizes to mobile and desktop"

Decorative Images

Images used purely for visual design — background textures, decorative dividers, abstract shapes — should use an empty alt attribute. This tells screen readers to skip the image entirely, which is correct behaviour. A decorative image has no informational value and describing it would create noise for screen reader users.

alt=""

Note: the alt attribute must still be present. Omitting it entirely causes some screen readers to read the filename as fallback. An empty alt="" is the deliberate, correct signal for "this image is decorative, skip it."

Functional Images

Images that act as buttons, links, or interactive controls should describe the action they perform, not the visual appearance of the image.

alt="download the image optimisation checklist PDF"

The test: if you removed the image and replaced it with a text link, what would the link text say? That's your alt text.

Best Practices for Writing Image Alt Text

Be specific and accurate. Describe what is actually in the image, not a vague category. "developer" is weaker than "frontend developer reviewing image compression settings in a browser dev tools panel."

Keep it concise. Most alt text should be under 125 characters. Screen readers typically truncate longer descriptions. If an image is genuinely complex enough to require more explanation, consider a longer description in the surrounding text and use the alt text to summarise.

Include relevant keywords naturally. Alt text is a legitimate SEO signal — but only when the keyword reflects what the image actually shows. If the image depicts a WebP conversion workflow, "WebP conversion workflow" is accurate alt text. If the image is unrelated to WebP but you include the keyword anyway, that's keyword stuffing.

Do not repeat the surrounding caption. Screen readers read both the alt text and the caption. Repeating the same information forces users to hear it twice.

Match the alt text to the image's role on the page. The same photograph of a compressed image might warrant different alt text on a general compression guide versus a specific WebP comparison article. Context shapes what details are worth describing.

Common Alt Text Mistakes

Keyword Stuffing

alt="image seo optimization seo images website images seo best images seo"

Search engines detect unnatural keyword repetition and treat it as a negative signal, not a positive one. Alt text stuffed with keywords also creates a terrible experience for screen reader users, who hear the repetition read aloud.

Generic or Empty Descriptions for Meaningful Images

alt="image"
alt="photo"
alt="screenshot"

These provide no useful information to search engines or screen readers. If the image is meaningful, the alt text needs to describe it.

Omitting Alt Text Entirely

An <img> without any alt attribute causes some screen readers to read the full filename or URL as fallback — often something like "snxcrjapcpejlxuhfjum dot supabase dot co slash storage slash image dash 17713884 dot webp." Always include the attribute, even if the value is empty for decorative images.

Describing Appearance Instead of Meaning

alt="blue rounded rectangle with white text"

For a button or functional image, this describes how it looks rather than what it does. A screen reader user needs to know what activates when they interact with the element, not what colour it is.

For a complete breakdown of image-level mistakes that affect both SEO and performance, the top image optimisation mistakes developers make covers alt text alongside compression errors, format choices, and responsive image failures.

Alt Text in a Complete Image Optimisation Workflow

Alt text works best as part of a broader image optimisation workflow rather than a last-minute addition. A practical sequence for developers and content teams:

1. Export images from design tools at the correct dimensions
2. Compress images using a browser-based tool — lossy for photos, lossless for graphics
3. Convert images to WebP or AVIF for modern format efficiency
4. Upload optimised images to the CMS or repository
5. Add descriptive, accurate alt text at upload time — not retroactively
6. Implement loading="lazy" on all offscreen images
7. Define explicit width and height to prevent layout shift

Adding alt text at the point of upload (step 5) rather than retroactively auditing it later prevents the accumulation of undescribed images that becomes increasingly difficult to fix at scale. The complete beginner guide to lazy loading images covers step 6 in depth for developers who want to implement it correctly rather than cargo-culting the loading="lazy" attribute.

For the full technical treatment of all ten optimisation techniques together, the SEO image optimisation checklist for developers covers alt text alongside format selection, compression, responsive images, and Core Web Vitals — as an integrated workflow rather than a list of disconnected steps.

The Future of Alt Text in AI Search

AI-powered search is changing how images are surfaced and used in search results. Image content increasingly appears inline within AI-generated answers, not just in a separate image tab. For images to be selected for these placements, search engines need high-confidence understanding of what the image depicts — and alt text remains a primary confidence signal even as visual AI improves.

The relationship between text-based image signals and AI search visibility is covered in depth in how images appear in AI search results, which explains specifically how AI models use contextual signals including alt text when selecting images for generative answer experiences.

The practical implication is that alt text will likely become more important, not less, as AI search matures — because precise textual descriptions help AI models validate and contextualise their visual interpretations with greater confidence.

Final Thoughts

Alt text is not a box to tick at the end of a project. It is a dual-purpose signal that simultaneously tells search engines what your images mean and tells users who cannot see them what they are missing.

Well-written alt text contributes to stronger image SEO, more accurate image search indexing, accessibility compliance, and a better experience for every user who encounters your content. The investment is small — a few seconds per image — and the compounding SEO and accessibility benefit over hundreds of images is significant.

The goal is the same whether you are writing for search engines or for screen reader users: describe what the image actually shows, clearly and specifically, in a way that communicates its meaning to someone who cannot see it.