[rodelrod]

This is mostly correct, but I don't see how it contradicts my statement.

Did the 646 standard account for variable-width characters at all?

[TheRealPomax]

Not so much contradict as wondering about the claim that it should be a specific Unicode codepoint when Unicode wasn't around when we started "computering" text (and the Académie Française can't have possibly formally declared things in terms of Unicode =)

What are the actual official rules in this case (and are there links to those? Because that'd be fascinating information to read through)?

[rodelrod]

Best reference I could find is here: https://www.lalanguefrancaise.com/articles/espace-insecable

Actually before the ":" specifically there should be a regular non-breaking space, not a narrow one. Except in Switzerland. Other punctuation marks take the narrow non-breaking space.

[TheRealPomax]

Love how it's "recommandé", "pour des raisons esthétiques."

Which I guess means we're completely free to ignore it. Pour mêmes raisons =P