When used as an adjective, both variants "descendent" and
"descendant" are acceptable, but when used as a noun only
"descendant" should be used.
For a reference, see here:
https://en.wiktionary.org/wiki/descendent#Noun
A feature can now be required only for specific input or font encodings:
- <feature>=enc1;enc2... Require the feature <feature> only if the
character is used in one if the specified font
or input encodings.
- <feature>!=enc1;enc2... Require the feature <feature> only if the
character is used in a font or input encoding
that is not among the specified.