CSS: font-variant-alternates: character-variant

Hi there,
I use a style alternative of an Opentype font for my printed materials. The type designer tells me that I can activate the style alternative via CSS in the corresponding web font. I guess it’s “font-variant-alternates: character-variant(user-defined-ident)” what he’s referring to? How/Can I implement this in Blocs?
Kind regards and merci!