The font-variant-alternates CSS property controls the usage of alternate glyphs. These alternate glyphs may be referenced by alternative names defined in @font-feature-values.
The @font-feature-values at-rule can be used to associate, for a given font face, a human-readable name with a numeric index that controls a particular OpenType font feature. For features that select alternative glyphs (stylistic, styleset, character-variant, swash, ornament or annotation), the font-variant-alternates property can then reference the human-readable name in order to apply the associated feature.
This allows CSS rules to enable alternative glyphs without needing to know the specific index values that a particular font uses to control them.