I don't think there is a technical reason for disallowing variation selectors after any starters (ccc=000); the normalization algorithm doesn't care about the general category of characters.
Mark On Sun, Feb 2, 2020 at 10:09 AM Richard Wordingham via Unicode < unicode@unicode.org> wrote: > On Sun, 2 Feb 2020 07:51:56 -0800 > Ken Whistler via Unicode <unicode@unicode.org> wrote: > > > 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. > > I can understand that for non-starters. However, a lot of non-spacing > combining marks are starters (i.e. ccc=0), so they would not be a > problem. <starter, variation selector> is an unbreakable block in > canonical equivalence-preserving changes. Is this restriction therefore > just a holdover from when canonical equivalence could be corrected? > > Richard. >
