Breakpoints
Two container-size breakpoints govern the layout, and both answer the form's own width rather than the viewport's — a form in a narrow sidebar behaves correctly on a wide screen. Below 52rem an fs-cols group collapses to a single column; below 32rem labels move above their controls. Resize the window to watch the sections above cross both. The lengths can be moved in site CSS — the vocabulary spec's Breakpoints section has the recipe.