Feature Request: Support WebTransport API in workerd [Tracking Issue]

Summary

Implement the WebTransport API in workerd to enable local development of Cloudflare Workers that use incoming WebTransport connections.

Note: This is a tracking issue to gauge interest and track progress. WebTransport may not yet be on the Workers runtime roadmap.

Motivation

WebTransport is a W3C Working Draft that provides reliable and unreliable data transport over HTTP/3 (QUIC). It offers several advantages over WebSockets:

• Head-of-line blocking elimination — QUIC's stream multiplexing avoids TCP head-of-line blocking
• Bidirectional streams — native support for ordered/unordered streams in both directions
• Datagrams — support for unreliable, unordered message delivery
• Lower latency — due to QUIC's handshake and connection migration capabilities

Browser support is strong: Chrome 97+, Firefox 114+, Edge 98+, and Safari (Technology Preview). The spec is targeting W3C Recommendation status in Q2 2026.

This issue tracks interest in WebTransport support for workerd/wrangler. This feature request is specifically about supporting incoming WebTransport connections in workerd for local development with wrangler.

Proposed Solution

Implement the WebTransport API surface in workerd:

1. WebTransport — the main constructor for incoming WebTransport sessions
2. WebTransportBidirectionalStream — bidirectional streams (readable + writable)
3. WebTransportDatagramDuplexStream — unreliable datagrams
4. WebTransportSendStream / WebTransportReceiveStream — unidirectional streams

This would allow developers to test WebTransport-based Workers locally via wrangler dev, mirroring how WebSocket support is already implemented.

Technical Considerations

• WebTransport requires HTTP/3 (QUIC) at the transport layer
• workerd would need to accept HTTP/3 connections with WebTransport extension negotiation
• The SEC-WEBSOCKET handler pattern used for WebSocket support could serve as a reference
• Consider whether this requires a compat flag (e.g., webtransport)

References

• W3C WebTransport Specification
• Can I Use: WebTransport
• Cloudflare Workers Protocols
• Cloudflare MoQ (related QUIC/HTTP3 infrastructure)

Priority

This would unblock developers building real-time applications (gaming, live collaboration, IoT) who want to use workerd for local development before deploying to Cloudflare's edge.