Emoji Variation Sequences: relaxing VS15/16

[Christoph Päper]

While VS1–VS14 (and VS17–256) are used for more or less arbitrary variant selection (VS4–VS14 are actually still unused), VS15 and VS16 have conventional de-facto semantics: select text style or emoji style. StandardizedVariants.txt explicitly claims, referring to Section 23.4 of Unicode 9, that implementations must ignore VSs that don’t form a standardized or ideographic variation sequence <http://unicode.org/Public/UCD/latest/ucd/StandardizedVariants.txt>:

> Standardized variation sequences are defined in this file.
> Ideographic variation sequences are defined according to the registration
> process specified in UTS #37, and are listed in the Ideographic
> Variation Database. Only those two types of variation sequences
> are sanctioned for use by conformant implementations.
> In all other cases, use of a variation selector character does
> not change the visual appearance of the preceding base character
> from what it would have had in the absence of the variation selector.
> 
> For more information on standardized variation sequences, 
> see Section 23.4, Variation Selectors, 
> in The Unicode Standard, Version 9.0.


Can this be relaxed for VS15 and VS16? 
Unlike VS1–VS3 they don’t operate on arbitrary glyph differences but on the actual Unicode property `Emoji_Presentation`. <http://www.unicode.org/Public/emoji/4.0/emoji-data.txt>