text/cache-manifest

A grave marker in the registry: this type served HTML5 Application Cache manifests, the first serious attempt at offline web apps, where a page pointed at a .appcache file beginning with the line CACHE MANIFEST and the browser hoarded the listed resources. The design's footguns — stale caches that refused to update, a manifest that cached itself — made it notorious among the developers who tried it, and Service Workers replaced it wholesale: the feature was cut from the WHATWG HTML standard by the end of 2020 and stripped from Firefox 85 and Chrome 95 around 2021. The type survives here because registries never forget; nothing should send it anymore.

.appcache was the convention; early tutorials used .manifest, which collided with other formats.

Defined by Cache manifest in HTML5 — Wikipedia · IANA registration. Registry facts from the IANA media-types registry via mime-db.