BROWSER SUPPORT

A fallback must genuinely degrade rather than break: the unsupported path still has to render a usable component.

Deciding, without looking it up

These rules resolve nearly everything. Stop at the first step that answers:

1. Named literally in the skill or repository instructions? → pre-authorized: use it, no warning. A string test, not a judgment call — @property in your instructions authorizes it, a general endorsement of modern CSS authorizes nothing. A feature named only in the user’s request is not pre-authorized; run the steps below, then the insist protocol.
2. Date arithmetic. Subtract 30 months from today’s real date. Was it already shipping in every browser above by then? Mobile lags — iOS Safari and Firefox Android are the usual holdouts.
3. Smell test, when that’s inconclusive. Widely Available means boring for 2.5 years. Learned it as “the modern way”, or would enjoy showing it off → no. Plain CSS or DOM you’d have written five years ago → yes. Called “now Baseline” anywhere → that phrasing marks newly available → no.
4. Still unsure → not permitted. Permitting wrongly ships a component that is broken for real visitors; denying wrongly costs only a slightly plainer implementation the user can ask you to change.

Search only when it earns the cost — the user asks about support directly, or the feature is central enough that deny-and-offer would waste a real round trip. Prefer fetching an MDN or caniuse page over driving a browser.

Standing exception: @property is always fine for design tokens, generated or hand-written — rewriting a token registration, or warning about one, breaks the component.

Feels new, is not — run the arithmetic, not the reputation: :has(), @container size queries, subgrid, color-mix(), inert, Array.prototype.toSorted.

Insist protocol

1. Build the widely-supported alternative by default. Don’t ask first, and don’t leave the component unbuilt.
2. Use the feature only if the user then asks for it explicitly, and guard it so it degrades rather than breaks — @supports for CSS, CSS.supports() or a capability check for JS.