[SPEC] Backwards-compatible metadata in Gemini

[Stephane Bortzmeyer]

On Mon, Feb 22, 2021 at 11:00:29PM +0100,
 text at sdfeu.org <text at sdfeu.org> wrote 
 a message of 40 lines which said:

> https://schema.org/ comes to mind:

The good point is that it already exists so we don't reinvent the
wheel. The not-so-good one is that schema.org is often
complicated. For instance, dates are expressed in the ISO 8601 format
which is 1) not publically available 2) very complicated (unlike what
most people think).

=> gemini://gemini.bortzmeyer.org/rfc-mirror/rfc3339.txt RFC 3339 is a more reasonable subset of ISO 8601

> as per gemini://gemini.circumlunar.space/docs/gemtext.gmi that states:

[Warning: the authoritative source is <gemini://gemini.circumlunar.space/docs/specification.gmi>]

> Anything which comes after the ``` characters of a line which toggles
> preformatted line *on* (i.e. the first, third, fifth, etc. toggling lines
> in a document) may be treated as "alt text" for the preformatted
> content.

Using this "alt text" feature is tempting, and it allows the metadata
proposal to avoid any change in the specification. But, currently, as
far as I know, there is no registry of possible alt texts. (I don't
ask for an official registry in the spec. Any widely recognized
registry will do.)

> ```purl.org/dc/terms/
> created: 2021-02-22

For dates, purl is even worse, allowing many format of dates, besides
the complicated ISO 8601. Laxity is bad for simplicity.