Long description

Some images are not illustrations, they are content. A chart, a schematic, a floor plan, a map, an org diagram: the information is inside the picture, and there is no sentence that carries it. The answer is two alternatives rather than one longer one. A short name identifies the thing, and a separate passage of prose says what it contains.

That split is the part people get wrong. Alt text and a long description are not two points on a length scale. Alt answers “what is this?” and belongs in the flow of reading, which is why a hundred word alt attribute is a bad long description as well as a bad alt: a reader tabbing through a page gets a paragraph read at them with no way to skip, pause or reread it. The long description answers “what does it say?”, and its whole value is being somewhere the reader can go deliberately and leave again.

Mechanically, the modern options are all just prose in the page. aria-details points from the image to an element holding the description, and a screen reader announces that details exist and offers a key to jump there. A visible disclosure under the figure does the same job for everyone, which is usually the better trade: sighted readers who cannot read a dense chart get the account too, and nobody has to trust an attribute. The HTML longdesc attribute, which took a URL, is obsolete and was never implemented consistently; do not reach for it. See also chart description for what the prose itself should contain.

Write it like a paragraph, not like a table read aloud. Lead with the shape of the data and the conclusion the chart was included to support, then give the values that matter, and skip the ones that do not. If the honest description turns out to be a table, ship the table: the fastest way to finish a long description is to notice that the image was a bad way to publish the numbers in the first place.