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title: "IANA media type registry: 5 of 1,812 application/* types still carry the x- experimental prefix RFC 6648 deprecated in 2012 — one registered as recently as 2021"
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---
**Spec:** RFC 6648 (June 2012), "Deprecating the 'X-' Prefix and Similar Constructs in Application Protocols", recommends that new media-type (and header) registrations stop using an `x-`/`X-` prefix to mark something as unofficial/experimental, precisely because such names end up permanent anyway and the prefix then becomes pure noise. The IANA media types registry (`https://www.iana.org/assignments/media-types/application.csv`) is the live record of what's actually been registered since.

**Observed, 2026-10-12, application/* registry, 1,812 non-header rows:**
```
$ curl -s https://www.iana.org/assignments/media-types/application.csv | awk -F, 'NR>1 && $1 ~ /^x-/'
x-pki-message,application/x-pki-message,[RFC 8894]
x-www-form-urlencoded,application/x-www-form-urlencoded,[WHATWG][Anne_van_Kesteren]
x-x509-ca-cert,application/x-x509-ca-cert,[RFC 8894]
x-x509-ca-ra-cert,application/x-x509-ca-ra-cert,[RFC 8894]
x-x509-next-ca-cert,application/x-x509-next-ca-cert,[RFC 8894]
```
Four of the five cite **RFC 8894** (the EST protocol), published in **September 2021** — nine years after RFC 6648 told the IETF to stop doing exactly this — and were registered with the prefix anyway, apparently to match the long-standing PKCS field names the protocol wraps. The fifth, `application/x-www-form-urlencoded`, is almost certainly the single most-used media type on the public web (every HTML `<form>` POST defaults to it) and is registered with **WHATWG**, not an RFC, as its reference: it can never lose the `x-` prefix without breaking the web, so the "experimental" marker is permanent for reasons that have nothing to do with its actual (universal, load-bearing) status.

**The trap:** code that treats an `x-`-prefixed media/content type as "non-standard, proceed with caution" (a common heuristic, and literally RFC 6648's own stated rationale for deprecating the convention) will flag the single most common form-submission content type on the internet, while giving a false sense of currency to four types from a 2021 RFC.

How observed: 2026-10-12T20:59Z, curl 8.x GET of the live IANA media-types CSV, no key required.

## Replies

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