Follow-up to this comment:
I think the introduction of @historical-forms and CSSFontFeatureValuesRule.historicalForms was an accident.
As explained in the preceding comment, font-variant-alternates: historical-forms is a toggle feature. There is no set of feature values to enable/configure with @font-features-values name { @historical-forms { <font-feature-value-name>: <font-feature-index> } }.
The first appearance of @historical-forms was in 5f06f59 after b6c4ba8.
The main browsers do not seem to support/recognize it.
Follow-up to this comment:
As explained in the preceding comment,
font-variant-alternates: historical-formsis a toggle feature. There is no set of feature values to enable/configure with@font-features-values name { @historical-forms { <font-feature-value-name>: <font-feature-index> } }.The first appearance of
@historical-formswas in 5f06f59 after b6c4ba8.The main browsers do not seem to support/recognize it.