Why does 'x-www-form-urlencoded' begin with 'x-www', when other standard content types do not? - http

Question

I understand that in the past, it was standard for custom headers names to use the prefix "X-" (I'm aware it no longer is considered standard to do this), but I've been unable to find if there is any relationship between this naming convention and the value ("application/x-www-form-urlencoded"). Did it start out as a custom content-type value that was later adopted or something?
I found this link here, which certainly was interesting, but have been unable to find the answer to my question.
Does anybody know the reason this prefix was chosen, and what it signifies?

Answer 1

it was standard for custom headers names to use the prefix "X-"
Actually … no, not at all. To be precise: It has never been a standard, just a best practice. It allowed implementors to introduce new content types and codings without the need to write an entire RFC for it. Nowadays the IANA Media Type Registry is good for that. RFC 6648 put an end to this practice.
The reason application/x-www-form-urlencoded is prefixed in this way (it is listed as a proper MIME type in said registry, btw)) stems from the fact that it is a "custom" method of structuring the query string in a URL. That part has never seen proper regulation. The people behind HTML just went and did it, which fully justified the prefix.

Answer 2

As far as the history: it has the x- prefix because it originated in a proposal from Mosaic—and since it was just a proposal, they used that x- extension prefix to initially define it. But then other browsers implemented it that way too, and nobody ever got around to taking the time to properly standardize an unprefixed alternative, so it just stuck that way, and here were are now.
It can be traced back to a 1993 thread on the www-talk mailing list titled “Submitting input-form data to server”, and in that thread, a September 1993 message from Marc Andreessen:
This is what we're doing in Mosaic 2.0… See
http://www.ncsa.uiuc.edu/SDG/Software/Mosaic/Docs/fill-out-forms/overview.html
...for details on what we're up to
That link is broken now but the document, titled “Mosaic for X version 2.0 Fill-Out Form Support” is archived at archive.org. Here’s the relevant excerpt:
ENCTYPE specifies the encoding for the fill-out form contents. This attribute only applies if METHOD is set to POST -- and even then, there is only one possible value (the default, application/x-www-form-urlencoded) so far.
Anyway, application/x-www-form-urlencoded is now formally defined in the URL spec, with algorithms for parsing and serializing it—though the section it’s all defined in has this note:
The application/x-www-form-urlencoded format is in many ways an aberrant monstrosity, the result of many years of implementation accidents and compromises leading to a set of requirements necessary for interoperability, but in no way representing good design practices. In particular, readers are cautioned to pay close attention to the twisted details involving repeated (and in some cases nested) conversions between character encodings and byte sequences. Unfortunately the format is in widespread use due to the prevalence of HTML forms.