Combining Marks and Variation Selectors

[Ken Whistler via Unicode]

Richard,

What it comes down to is avoidance of conundrums involving canonical 
reordering for normalization. The effect of variation selectors is 
defined in terms of an immediate adjacency. If you allowed variation 
selectors to be defined for combining marks of ccc!=0, then 
normalization of sequences could, in principle, move the two apart. That 
would make implementation of the intended rendering much more difficult.

That is basically why the UTC, from the start, ruled out using variation 
selectors to try to make graphic distinctions between different styles 
of acute accent marks explicit, for example.

--Ken

On 2/1/2020 7:30 PM, Richard Wordingham via Unicode wrote:
> Ah, I missed that change from Version 5.0, where the restriction was,
> 'The base character in a variation sequence is never a combining
> character or a decomposable character'.  I now need to rephrase the
> question.  Why are marks other than spacing marks prohibited?
>