WebTransport send groups and send-stream prioritization

Category: Network / Connectivity
Type: Chromium catches up
Status: Proposed (Chrome Proposed)
Intent stage: None

Breaking change: No
Enterprise impact: None
Chromium flag: enable-experimental-web-platform-features
Spec maturity: Published standard
Web Platform Tests: Yes

WPT details: https://wpt.fyi/results/webtransport/sendgroup.https.any.html https://wpt.fyi/results/webtransport/sendorder.https.any.html https://wpt.fyi/results/webtransport/sendstream-bad-chunk.https.any.html https://wpt.fyi/results/webtransport/send-stream-stats.https.any.html

Summary

WebTransport already lets sites send data through WritableStreams. This feature lets applications assign outgoing streams to send groups and use sendOrder to prioritize streams within each group. It also provides sending statistics for streams and groups while preserving WritableStream transfer behavior.

Motivation

Real-time applications often send multiple independent flows over one WebTransport session. When those flows compete for bandwidth, applications need to keep one flow from dominating while deciding which streams within each flow should go first. They also need visibility into sending progress so they can understand how those flows are performing.

Standards & signals

Specification: https://www.w3.org/TR/webtransport/#webtransportsendstream
Firefox: Shipped/Shipping — Firefox exposes WebTransportSendStream as a WritableStream subclass for outgoing WebTransport streams, including streams created by createUnidirectionalStream(). It added WebTransportSendGroup and createSendGroup() in Firefox 155, released September 1, 2026; group assignments are passed to its network implementation - https://www.firefox.com/en-US/firefox/155.0/releasenotes/ Firefox does not yet provide equivalent statistics behavior. In its current source, WebTransportSendStream.getStats() rejects with NotSupportedError.
Safari: Shipped/Shipping — WebKit implements WebTransportSendGroup, WebTransportSendStream, sendGroup/sendOrder, both statistics methods, transferability, and the specialized writer. Safari 26.4 includes these WebTransport additions.
Web developers: Positive — The Interop 2026 WebTransport proposal received 28 positive reactions. Developers also reported active WebTransport use and significant cross-engine differences.
Tracking bug: https://crbug.com/568825278

Ecosystem

Web Platform Tests: https://wpt.fyi/results/?q=webtransport%2Fsendgroup
Chrome Platform Showcase: https://chrome-platform-showcase.paulkinlan-ea.deno.net/features
GenDN reference: https://gendn.paulkinlan-ea.deno.net/features
chrome.dev demos search: https://www.google.com/search?q=site%3Achrome.dev+WebTransport%20send%20groups%20and%20send-stream%20prioritization
MDN Web Docs: https://developer.mozilla.org/en-US/search?q=WebTransport%20send%20groups%20and%20send-stream%20prioritization
Baseline status: https://webstatus.dev/?q=WebTransport%20send%20groups%20and%20send-stream%20prioritization

Explainers: https://github.com/w3c/webtransport/blob/main/explainer.md#6-sending-video-one-stream-per-frame-or-segment-with-send-order https://github.com/w3c/webtransport/blob/main/explainer.md#7-sending-real-time-game-data-per-stream-with-send-order https://github.com/w3c/webtransport/blob/main/explainer.md#8-managing-bandwidth-with-send-groups

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