Alt text (alternative text) is an HTML attribute that provides a text description of an image. It is read aloud by screen readers, displayed when images fail to load, and indexed by search engines. Writing good alt text is a skill that takes practice, but the core principle is simple: describe what the image shows in a way that conveys the same information a sighted user would get from seeing it.

The alt attribute is added to the img HTML element: img src='product.jpg' alt='Red leather wallet with card slots and coin pocket, shown open on white background'. This description tells a screen reader user exactly what the image shows without requiring them to see it.

Alt text should be concise but complete. The WCAG guidelines recommend keeping alt text under 150 characters when possible, though longer descriptions are acceptable for complex images. Avoid starting with 'Image of' or 'Photo of' — screen readers already announce that they are reading an image. Just describe the content directly.

Not all images serve the same purpose, and alt text should reflect the image's function rather than just its appearance. Understanding the different types of images helps you write more effective alt text.

Informative images convey information that is not available in the surrounding text. These need descriptive alt text that communicates the information. A chart showing sales growth needs alt text that describes the trend: 'Bar chart showing 40% revenue growth from Q1 to Q4 2025.' A product photo needs alt text describing the product's appearance and key features.

Decorative images serve a purely aesthetic purpose and do not add information. These should have empty alt text (alt='') so screen readers skip them entirely. A decorative background pattern or a generic stock photo used purely for visual interest does not need description — describing it would add noise without value.

Functional images — images that serve as links or buttons — need alt text that describes the function, not the appearance. A magnifying glass icon that triggers a search should have alt='Search', not alt='Magnifying glass icon.' The user needs to know what will happen when they activate it, not what it looks like.

Complex images like charts, graphs, and infographics often require more than 150 characters to describe adequately. For these, provide a brief alt text summary and a longer description in the surrounding text or in a linked description page.

Keyword stuffing is the most common alt text mistake. Writing alt text like 'image resize tool free online image resizer best image resizer 2026 resize images free' is a spam signal that Google penalizes. Alt text should describe the image accurately and naturally — if keywords fit naturally, great, but they should never be forced.

Redundant alt text is another common mistake. If the image caption or surrounding text already describes the image, the alt text does not need to repeat that information. A photo of a sunset with a caption reading 'Sunset over the Pacific Ocean' does not need alt text that says 'Sunset over the Pacific Ocean' — that would be read twice by screen readers. Instead, use empty alt text (alt='') to indicate the image is decorative in this context.

Vague alt text like 'photo' or 'image' or 'picture' provides no useful information. Every image that conveys information should have specific, descriptive alt text. Similarly, using the file name as alt text (alt='IMG_4892.jpg') is unhelpful and should be avoided.

Over-describing is also a problem. Alt text should convey the essential information, not every detail. A product photo does not need alt text describing the lighting, the angle, the background texture, and every minor detail — focus on the information that matters to the user.

Image file names are an often-overlooked aspect of image accessibility and SEO. Search engines use file names as one signal for understanding image content. A file named IMG_4892.jpg tells Google nothing. A file named red-leather-wallet-open-white-background.jpg tells Google exactly what the image shows.

Descriptive file names also help with organization and workflow. When you have hundreds of product images, descriptive names make it much easier to find specific images later. Use hyphens to separate words (not underscores or spaces), keep names concise but descriptive, and include relevant keywords naturally.

For images that will be used across multiple contexts — on your website, in social media posts, in email campaigns — a consistent naming convention makes asset management much easier. A convention like product-name-color-angle.jpg (red-wallet-front.jpg, red-wallet-open.jpg, red-wallet-detail.jpg) creates a logical, searchable library.

The Web Content Accessibility Guidelines (WCAG) 2.1 include several success criteria related to images. Success Criterion 1.1.1 (Non-text Content) requires that all non-text content has a text alternative that serves the equivalent purpose. This is the foundation of image accessibility — every informative image needs alt text.

WCAG Level AA compliance — the standard required by most accessibility laws and regulations — requires meeting all Level A and Level AA success criteria. For images, this means providing alt text for all informative images, using empty alt text for decorative images, and providing longer descriptions for complex images like charts and infographics.

WCAG 2.2, released in 2023, added new success criteria but did not significantly change the requirements for images. The core principle remains: provide text alternatives that convey the same information as the image. WCAG 3.0 is in development and will introduce a new conformance model, but the fundamental accessibility requirements for images are unlikely to change dramatically.

For most websites, achieving WCAG Level AA compliance for images requires three things: writing descriptive alt text for all informative images, using empty alt text for decorative images, and providing extended descriptions for complex images. These are achievable goals that significantly improve the experience for users with visual impairments.