Use of tag characters in a private encoding - is it valid please?

[James Kass]

On 2024-05-02 9:05 PM, Asmus Freytag via Unicode wrote:
> PS: you are free to solicit other parties to join such private 
> agreements and you may even choose to write them down. However, it's 
> up to you to resolve any issues due to non-compliance with your 
> private agreements. Unicode doesn't care -- as long as you don't agree 
> to things that conflict with conformance to the Standard. In which 
> case, such any conformance by participants in your agreement may no 
> longer be valid.

Wouldn’t this kind of private use agreement be considered a higher level 
protocol?

[HTML]
Yadda yadda <img src="aardvark.jpg"> et cetera.

[tags shown using encircled alphanumerics]
Yadda yadda 🆔Ⓠ④⑥②①② et cetera.

There’s nothing stopping folks from putting out fonts with glyphs 
covering large sets of images using QID numbers expressed as tag 
characters (or even as enclosed alphanumerics) and treating them as 
ligature substitutions.  The same goes for any non-QID strings, as well.

Yet both of the examples above can be considered mark-up languages which 
use elements of text.  Which may explain why “Unicode doesn’t care” 
about such private agreements.  Because they are beyond the realm of 
plain-text.