WebSockets operate strictly over TCP, which enforces ordered, reliable stream delivery. If a single TCP packet is dropped on the network, the operating system kernel holds all subsequent packets in buffer until the missing packet is retransmitted and acknowledged (Head-of-Line blocking), causing massive latency spikes (50-300ms) that ruin real-time games, live cloud audio, and high-frequency trading. WebTransport runs over HTTP/3 and QUIC (UDP), providing two revolutionary capabilities: unreliable out-of-order datagrams that never retransmit or block, and multiplexed streams where packet loss on stream A never stalls stream B.

WebTransport datagrams (accessible via transport.datagrams.writable and transport.datagrams.readable) transmit individual datagrams directly within QUIC DATAGRAM frames (RFC 9221). Because QUIC does not buffer or reorder datagrams, an application receives packets the exact instant they arrive over the wire. If a packet is lost in transit, the network stack simply discards it and passes the next newest packet immediately to userspace with zero milliseconds of delay.

WebTransport provides three distinct communication primitives over a single QUIC connection: 1) Datagrams: Unreliable, unordered, MTU-bounded (~1,200 bytes) packets ideal for gamepad inputs, player orientation, and ephemeral telemetry; 2) Unidirectional Streams: Reliable, ordered byte streams flowing in one direction, perfect for file chunks or video slices; 3) Bidirectional Streams: Reliable full-duplex client-server streams that replace HTTP request-response or RPC with zero connection setup overhead.

While WebRTC DataChannels also support unreliable UDP-like transport via SCTP-over-DTLS, WebRTC was designed primarily for peer-to-peer mesh networking. Setting up a WebRTC DataChannel with a central media server requires complex SDP offer/answer signaling, ICE candidate gathering, STUN/TURN traversal, and heavy protocol stacks. WebTransport connects directly to any HTTP/3 server with a simple HTTPS URL (new WebTransport("https://server.com:4433/stream")), uses standard Web PKI certificates, integrates seamlessly with HTTP infrastructure, and supports server pooling.