he said.. in courier, monospaced paragraphs format, morally as close to "plaintext" as you can be with a couple of diagrams which could have been ASCII art...
I don't see why pure plain text is better in any way than plain text with formatting, like a simplified form of HTML (<a>, <b>, <sup>, some kind of table formatting, etc). The latter is non-proprietary, easily read and diffed, and communicates better than pure text.
Images have their own value, as do animations and video on occasion. Here matters become more complicated - image formats are generally non-human-readable and non-diffable (though SVG or a similar format could solve those problems for schematic-type images) and image conversions generally involve data loss. For starters, though, one should at least use a non-proprietary format for images and video.
Here text/plain is the only that works for e.g. my mother.
Yes, but, the problem isn't typically being proprietary, when it comes to future use, but a closed, non standard, unknown format.
Yet you're creating a new standard here, with your own rules, which no one will understand, and which no automated tools can convert to another format.
(Eg some kind of table formatting)
Better to be 100% html than this.
(Maybe you meant that, but regardless, this is a good place for me to comment on standards being more important than anything else.)
that's a bit harsh.
And frankly, I don't understand 100% english either, but still we use it to communicate.
Standards exist for a reason.
We already have issues with people not understanding specs, and writing data out of spec, even with that spec in RFC's!
Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.
https://datatracker.ietf.org/doc/html/rfc3339#section-5.6
But what harm does it to restrict oneself to a few html tags?
If they had needed to convey an image or contextual information like some rich API spec, they would presumably have used something else.
On the serious side, ASCII art diagrams are splendid and I very often use them myself, though they can get quite complex and thus messy to maintain. There comes a certain point where they lose their simplicity, sadly.