Stylistic set

Most letters in a font have more than one drawing available. The double-storey a has a single-storey twin, the two-storey g a single-storey one, the l a version with a tail, the zero a version with a slash. A stylistic set is how a type designer bundles those alternates: ss01, ss02, up to ss20, each a numbered group that swaps in a coherent handful of shapes when you switch it on. The grouping is the point. A face’s “code” set might slash the zero, put a foot on the 1 and a tail on the l all at once, because those three decisions only make sense together, and no interface should make you turn them on one at a time.

The numbers carry no meaning. ss01 in one family is a single-storey a and in another it is old-fashioned quotes, because the OpenType specification reserves the tags and leaves their contents entirely to the designer. The only way to know what a set does is the family’s own specimen or documentation, which is why CSS gives the tag a human-readable wrapper: font-variant-alternates: styleset(code) reads the names out of the font’s own table, while font-feature-settings: "ss01" 1 just asks for the number. Character variants (cv01 to cv99) are the finer-grained cousin, one letter each rather than a bundle, for when you want the tailed l without the slashed zero.

Neighbouring features do related but distinct work, and the distinction is worth keeping straight. A ligature fuses two adjacent letters into one drawing to fix a collision, and it fires wherever that pair occurs. Contextual alternates swap a shape based on what sits beside it, automatically, with no decision from you. A swash is a specific kind of alternate, the ornate extended one, usually parked in its own swsh feature. A stylistic set is the only one of the four that is a deliberate editorial choice about the voice of the text: nothing is broken, nothing is contextual, you simply prefer the other a.

The trap is asking for a set the file does not have. There is no error, no warning, no fallback: the request is silently ignored and the default glyphs render, so a stylesheet full of "ss02" 1 can look exactly like a stylesheet without it. Check the face before the CSS, and be aware that most system fonts carry no sets at all. The subtler version of the same trap is delivery, because a face that has ten sets can arrive with none: the specimen above is set in Fira Code from the type designer’s own release, since the Google Fonts build of that family keeps the contextual rules and drops every stylistic set, so identical CSS against an identically named family does nothing at all depending on where the file came from.