ICO (x-icon)

image/x-icon

Extensions

.ico

Compressible

yes

Status

de facto

The name everyone actually uses for favicons, despite never being registered with IANA — the official type is image/vnd.microsoft.icon, but x-icon won the deployment war so thoroughly that the WHATWG sniffing standard identifies icon bytes as image/x-icon. Send either; browsers treat them identically. The format is a container of multiple icon sizes, and its four-byte header (reserved zeros, then type 1 for icon) doubles as its signature.

Magic bytes

00 00 01 00 · offset 0

Per WHATWG MIME Sniffing — image type patterns · test a file against it.

Defined by WHATWG MIME Sniffing Standard. Registry facts from the IANA media-types registry via mime-db.

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