Contextual alternates

Some letters look wrong next to certain others. A joining script whose exit stroke lands on the entry stroke of the next letter needs a different exit when the next letter is a tall one. A doubled letter drawn from one outline twice reads as a stamp rather than as handwriting. The calt feature is a font’s own answer to those cases: a table of rules saying which glyph to use when a particular neighbour is present, applied by the shaping engine with no decision from the person setting the text.

That automatic quality is the whole distinction from its neighbours in the OpenType vocabulary. A stylistic set is a deliberate editorial switch: you prefer the other a, and you say so. Contextual alternates fire on their own, wherever the context calls for them, and they are on by default in CSS, which is why most people never learn the tag exists. A ligature is close but not the same shape of rule: it fuses a pair into one glyph, while a contextual alternate keeps the count of glyphs and changes which drawing one of them uses. In practice the two overlap, and programming faces lean on calt heavily, both for arrows built out of - and > and for keeping repeated symbols from stacking into one blur.

The CSS is two spellings of the same request. font-variant-ligatures: contextual and no-contextual are the readable form, and font-feature-settings: "calt" 1 or 0 is the raw tag. Turning it off is the rarer and more interesting case: a code editor showing exactly the characters in the file, a diagram where -> must stay two characters wide, a table where a substitution would break the column. Otherwise leave it alone, because the type designer wrote those rules about their own drawings and knows where the collisions are.

The trap is the same one every OpenType tag carries: a request for a feature the file does not have is silently ignored, and most text faces have no calt table at all. Nothing warns you, so a script that looks mechanical is not evidence that the browser dropped the feature; it is usually evidence that the font never shipped one. The specimen above says so out loud: no face this site loads substitutes anything for calt, so the alternate is drawn on and the switch stands for the substitution.