Two things to look at. First, how the font behaves across its single weight axis (100 → 1000). Second, how each letterform variant — default, ss01, ss02, ss03 — animates differently when that axis moves, and what calt does on top.
One variable axis. The headline below loops between wght 200 and 900 so you can watch the outlines morph; the staircase pins five sample weights.
font-feature-settings: "calt" 0 — contextual swapping disabled, so you see only what the variant column explicitly requests.
Letter-spacing warning: any non-zero CSS letter-spacing on a specimen makes browsers silently disable contextual alternates — calt looks enabled but does nothing, and most of the animation disappears with it. Try it live: drag the letter-spacing slider in the sticky bar and watch the calt-on rows snap to their base glyphs. Tighter spacing therefore has to be drawn into the font's own metrics by the foundry — it can't come from CSS. The alternates are also drawn to pass close to the base shapes around wght 500, so judge calt at the weight extremes (200 / 900), not mid-axis.