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?
