Browser support

List browser families and versions, mobile requirements, assistive-technology coverage, and optional capabilities. Framework support does not automatically provide WebTransport, service workers, storage, workers, or WebAssembly everywhere.

Isolate capability selection:

function createRealtime() {
  if ('WebTransport' in window) return createWebTransport();
  if ('WebSocket' in window) return createWebSocket();
  return createPolling();
}

Detect the API used, not a browser brand. Keep fallback selection in one service so components consume domain events rather than transport differences.

Test production output: cold load, direct route entry, back/forward, storage denial, offline transition, reduced motion, keyboard navigation, and service-worker upgrade. Disable capabilities to test fallbacks even when CI supports them.

When no fallback exists, render an actionable unsupported state before constructing the service. A runtime exception is not a support policy.