Writing alt text that describes the image function rather than the image content.
A good alt text conveys the image's contribution within its specific context. Decorative images remain blank, while functional images describe their action.
"Writing alt text based on image function" is considered here from the perspective of "Captions, Language, and Content Alternatives." For web developers and UX teams, "contextual replacement information" and "motive inventory" are particularly important.
Published: 3 min read · Author: Sebastian Geier
How does alt text describe the function of an image instead of just its visible details?
The alternative text replaces precisely the information or action that would be lost in the current context if the image were missing. It does not repeat the caption or visible link text; decorative graphics remain empty, while functional images name the goal or action.
Intentional emptiness
Control signal
Signal 1
Percentage of editorially reviewed images with documented function and a corresponding alt text decision, including intentional emptiness.
Control signal
Signal 2
Number of linked or interactable graphics whose accessible name clearly indicates the goal or action without a visible image.
No redundancy
For each image, first determine whether it contributes information, action, mood, or is purely decorative.
Formulate the missing contextual information in a concise sentence or, in the case of genuine decoration, deliberately leave the alt text blank.
Review the page without images or in linear screen reader output and correct redundant or missing information.
Implementation example: "Motif inventory"
A diagram shows that the error rate increases after a release, while the image caption only mentions the time period and source. The alternative text describes this core message instead of colors and lines. The adjacent arrow icon receives an empty alt attribute because the link text "Download Report" already fully describes the action.
Motif inventory
Motif inventory – The text lists visible objects, even though the image, in context, explains a process, a result, or an action.
SEO text in the alt attribute – Keywords and brand claims lengthen the output without replacing the missing image information.
Empty function name – A linked icon is considered decorative and therefore leaves the purpose or action of the control unnamed.
Contextual replacement information
Test criterion
Contextual replacement information
The text conveys precisely the information, function, or consequence that would be missing without the image at this location.
Test criterion
No redundancy
The image caption, heading, and adjacent body text are not repeated in the alternative text.
Intentional emptiness – Purely decorative images have empty alternative text and therefore disappear from the content-related assistive technology output.
Which questions about "writing alt text according to image function" trigger further checks
A relevant follow-up question answered Reduce movements and animations for sensitive users"How are movements and animations safely reduced for sensitive users?"
A second link for "writing alt text based on image function" leads to Writing content for retrieval systems without sounding like machine languageThis contribution remains focused on the question, "How can content be made clear for retrieval without sounding unnatural to machines?"
If you want to practically implement "writing alt text based on image function," you can refer to Robust Website Systems This focuses on "Labels, language, and content alternatives" and "contextual replacement information."
Conclusion: Writing alt text based on image function
A good alt text replaces the image's role within the specific content, not its entire appearance. Decorative elements can remain silent, while functional and informative images require precise replacement information.
Sources and Further Information
The following sources document the technical and methodological guidelines used for "writing alt text based on image function."
Web Content Accessibility Guidelines (WCAG) 2.2 – W3CW3C recommendation with criteria for text alternatives, language of parts, link purpose, labels, and error identification.
Labeling Controls – W3C Web Accessibility InitiativeOfficial guidance on visible and programmatically linked labels for various form controls.
Key Thesis
The alternative text replaces the information or action that the image provides at that point. It does not repeat adjacent text or meaningless visual elements.
What This Is Not About
An alt text is neither an inventory list of visible objects nor a place for keywords. Purely decorative images do not require a spoken replacement.
What it's about
It replaces precisely the information or function that would be lost without the image in the respective context and remains empty if adjacent text or purely decorative elements already cover everything necessary.
Semantic HTML & Accessibility
Clearly labeling PDFs, downloads, and external links
"Writing Alt Text Based on Image Function" includes, as a separate checklist, the question: How can PDFs, downloads, and external links be marked in a clear and accessible way?
Semantic HTML & Accessibility
Using Semantic HTML as a Foundation for SEO and AI Understanding
Supplements "Write alt text according to image function" with a separate decision: How does semantic HTML support understanding by search and AI systems?
Insights Overview
All VELUNO Insights at a Glance
Further analyses on Website Systems, digital visibility, and robust working models.
No redundancy: a concrete checkpoint
An editorial image sample can separately mark the subject description, context function, and any existing adjacent text. This creates a reusable decision aid for informative, functional, and decorative cases.