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>
