WebTransport
Browser API over HTTP/3 + QUIC that gives you both bidirectional reliable streams and unreliable datagrams — like WebSocket + UDP in one package.
Designed as a successor to WebSocket for realtime apps: games, video, telemetry. Multiplexed streams avoid head-of-line blocking; datagrams give lower latency when loss is okay. Supports pooling, congestion control via QUIC, and requires HTTPS + cert (or a WebTransport-specific cert hash).
Stable in Chrome/Edge since 2022, Firefox since 2024, Safari implementing it (preview in 18). Common uses: cloud gaming, browser-based DAWs, multiplayer games needing UDP-like semantics.