What is alt text?
Alt text (short for alternative text) is a written description that represents an image for people who cannot see it. When someone uses assistive technology such as a screen reader, the alt text is what gets read aloud in place of the image. Without alt text, the image and the information it communicates becomes invisible to them.
If the image is decorative and does not contribute meaning, the image should be marked as decorative so screen readers skip it entirely.
Providing alt text is one of the simplest and most impactful ways to ensure your course materials are accessible to all learners.
Screen readers are assistive technologies used by people who are blind, have low vision, or have difficulty visually processing text. They read out loud what is on the screen, along with semantic information, like headings, links, buttons, tables, menus, text, and alt text for images.
When a screen reader reaches an image, it announces the alt text.
If no alt text has been provided, the screen reader may skip the image or announce a meaningless filename, leaving the learner unsure of what they missed.
Alt text is essential for a wide range of learners, not only people who are fully blind. A common misconception is that alt text only serves a small group, but in reality, many types of learners rely on it to understand visual content.
Alt text supports:
Learners who are blind
These learners use screen readers to access all course content. Without alt text, images convey no information at all. The screen reader may simply skip the image or speak its filename, leaving the learner unaware of what they missed.
Learners with low vision
Some learners can see large text but cannot make out smaller details in images. Alt text ensures they can still access the same information without needing to zoom excessively or rely on someone else to describe the image.
Learners with certain cognitive or processing disabilities
Some students experience difficulty interpreting complex visuals. A well-written alt text can offer a clearer, more direct explanation of what is important in the image, supporting comprehension.
Learners with temporary or situational limitations
Accessibility is not just about disability. Anyone can benefit from alt text when:
- images fail to load due to poor internet connections,
- they are using an older device or small screen,
- they are multitasking and relying on audio,
- or they are in a setting where visibility is reduced (sun glare, low light).
Learners who use text-only or assistive technologies
Some students may use text-only browsers, customised accessibility tools, or reading modes that suppress images but preserve alt text. Without alt text, the meaning of images is lost in these environments.
How to write alt text
Alt text is a brief description that communicates the meaning or purpose of an image. It does not need to describe every visual detail, only the information the learner needs to understand in the context of the page. We recommend that alt text should be kept under 200 characters.
If the image conveys important information, the alt text should make that information available to someone who cannot see the visual.
Why should alt text be contextual?
The right alt text depends on why the image is there.
The same photograph of a dog can require completely different alt text depending on what the page is trying to teach:
- If the image of a dog appears in a lesson on dog breeds, the important information might be the dog’s physical characteristics.
- If the image appears in a lesson on dog behavior, the dog’s posture or body language becomes the key information.
- If the dog is simply a decorative image accompanying a story about loyalty, then the dog’s appearance may not matter at all, so the correct choice is to mark it as decorative.
Alt text does not describe the image for its own sake. It conveys the purpose of the image within the lesson.
This contextual approach helps screen reader users receive information with the same intent and meaning as sighted learners.
Alt text for complex images
Some images contain more information than a short alt text can reasonably convey, such as graphs, charts, data visualizations, maps, or diagrams, and screenshots. Screen readers read out alt text all at once, so learners using them can’t navigate alt text like they can navigate other text within a page. For this reason, we recommend keeping the alt text brief, and providing more information about the image in the image caption, or the paragraph following the image.
For these cases:
- The alt text should give a high-level summary of what the image represents.
- A long description (placed in the surrounding text, a caption, or another nearby element) should convey the full details that sighted learners receive visually.
This two-layer approach ensures that screen reader users can access both the overview and the fine-grained information.
