The appropriate alt text depends on what an image communicates in its particular position. The same photograph may carry essential information in one article and provide atmosphere elsewhere. Its alternative cannot therefore be derived solely from a filename or a list of visible objects.
Start by asking what a reader would miss if the image were unavailable. The answer may concern a material property, an action or the destination of a linked symbol. Sometimes no additional information is lost because the image is decorative or nearby text already supplies what matters.
Describe informative content
When a photograph supports a substantive point, identify the relevant visible information. If an article compares the arrangement of two controls, its alternative should convey that difference. “Device on a table” would miss the purpose.
Do not invent characteristics the image cannot establish. A workshop photograph does not demonstrate certification or a particular standard of work. Assumed emotions or personal attributes should not appear without a relevant factual reason either.
The W3C Web Accessibility Initiative decision tree helps distinguish informative, functional and decorative uses. Apply it to the specific case rather than treating it as a formula for identical descriptions on every image.
Functional images need understandable purposes
If an image is the only label for a link or button, its accessible name needs to communicate the destination or action. A printer symbol in that situation represents printing, not merely the appearance of a small printer.
For an image card with a heading, inspect the complete link structure. If the picture and title form one link, duplicate detailed naming may be unhelpful. The implementation determines which information already contributes to its accessible name. Review the actual markup rather than judging only the text stored in an image manager.
A logo may identify the organization and, depending on its use, a route to the homepage. Its function in that position matters. A decorative background logo does not perform the same task as the only visible content of a Home link.
Handle decoration and complex information differently
An empty alternative can be appropriate for a purely decorative image. That is different from accidentally omitting the attribute. Make the decision deliberately so an unhelpful filename or substitute is not announced instead.
A chart containing several values often needs more than short alt text. Provide its important data or conclusions in ordinary page content as well. The alternative can identify the chart's subject in that context. Avoid hiding the entire explanation inside an extremely long attribute.
Necessary wording embedded in an image also needs an equivalent. Publishing it as normal text is often preferable because it remains enlargeable, translatable and easier to update.
Review the complete context
Read the paragraph, caption, link and alternative together. Is information unnecessarily repeated? Is the important point missing? Does the language match the rest of the page? Translated pages need translated alternatives as well as translated body text.
Check the real output with suitable assistive tools and inspect the technical association. A well-written sentence does little good when attached to the wrong image. The quality test is whether the image's function remains understandable, not whether the alternative reaches a particular length or contains a target number of keywords. Revisit the decision if a photograph later moves to a different page role.
