Intent to Ship: WebTransport reliability attributes

[Chromestatus]

Contact emails

ri...@chromium.org

Explainer

https://github.com/w3c/webtransport/blob/main/explainer.md#transport-modes

Specification

https://www.w3.org/TR/webtransport/#dom-webtransport-reliability

Summary

Adds the  WebTransport.reliability  instance attribute and static WebTransport.supportsReliableOnly attribute. These APIs indicate whether the user agent supports WebTransport over exclusively reliable connections and whether an established session supports unreliable transport such as datagrams. In Chromium, reliability initially returns "pending" and changes to  "supports-unreliable" after an HTTP/3 WebTransport connection is established.  Since Chromium does not currently support HTTP/2 fallback,  WebTransport.supportsReliableOnly returns false; any successfully established session uses HTTP/3 and supports unreliable datagrams.

Blink component

Blink>Network>WebTransport

Web Feature ID

webtransport

Motivation

WebTransport may operate over connections with different reliability capabilities. Applications such as games, interactive media, and real-time collaboration tools need to know whether unreliable datagrams are available before selecting their transport strategy. WebTransport.supportsReliableOnly  reports whether the user agent supports WebTransport over exclusively reliable connections. The per-session reliability attribute is "pending" while connecting and becomes "reliable-only" or  "supports-unreliable" once the transport is known. Chromium currently implements WebTransport over HTTP/3 with unreliable datagram support, so established sessions report "supports-unreliable" and  supportsReliableOnly  returns false.

Initial public proposal

https://github.com/w3c/webtransport/pull/385

TAG review

This is a small incremental implementation of attributes already defined in the existing W3C WebTransport specification. It does not introduce a new API design or Chromium-specific extension

TAG review status

Not applicable

Goals for experimentation

None

Risks

Interoperability and Compatibility

Compatibility risk is low because both attributes are additive and do not change existing WebTransport behavior. Interoperability risk is low because the attributes and their  "pending" ,  "reliable-only" , and  "supports-unreliable"  values are defined by the W3C WebTransport specification. Equivalent APIs are implemented or partially implemented in WebKit and Firefox, and WebTransport is an Interop 2026 focus area.

Gecko: Shipped/Shipping (https://bugzilla.mozilla.org/show_bug.cgi?id=1709355)

WebKit: Shipped/Shipping (https://github.com/WebKit/WebKit/pull/55124)

Web developers: Positive (https://github.com/web-platform-tests/interop/issues/1121) The Interop 2026 WebTransport proposal received 28 👍 reactions. Developers also reported active WebTransport use and significant cross-engine differences.

Other signals:

Ergonomics

Ergonomics risk is low.  reliability  synchronously returns  "pending"  while connecting, and applications can await  WebTransport.ready  before reading its final value. The static  supportsReliableOnly  attribute enables capability detection before creating a connection.

Activation

Activation risk is low. This is an additive API that does not change existing WebTransport behavior or require server changes. Developers can use standard feature detection and retain existing fallbacks for older browsers.

Security

No new network capability or sensitive information is exposed. The attributes report whether an implementation or session supports exclusively reliable or unreliable WebTransport, which is already observable through the available WebTransport APIs. They expose no server data, credentials, addresses, or user-specific information.

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

Low. This is an additive API surface and does not modify the behavior of existing WebTransport connections. WebView applications that do not use the attributes are unaffected

Debuggability

The attributes can be inspected directly from JavaScript or the Chrome DevTools Console. Developers can inspect  transport.reliability  before and after  transport.ready  resolves and inspect WebTransport.supportsReliableOnly  without creating a connection. No additional DevTools-specific support is required

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

Yes
All Blink platforms on which WebTransport is supported. The implementation is in cross-platform Blink code.

Is this feature fully tested by web-platform-tests?

Yes
WPT coverage verifies that: • Initial  "pending"  state. • Transition after  ready . • Web IDL exposure. Static attribute and initial state: https://wpt.fyi/results/webtransport/reliability.https.any.html?label=experimental&label=master&aligned State after connection establishment: https://wpt.fyi/results/webtransport/reliability.https.window.html?label=experimental&label=master&aligned IDL harness: https://wpt.fyi/results/webtransport/idlharness.https.sub.any.html?label=experimental&label=master&aligned

Flag name on about://flags

chrome://flags/#enable-experimental-web-platform-features

Finch feature name

WebTransportReliability

Rollout plan

Will ship enabled for all users

Requires code in //chrome?

False

Tracking bug

https://issues.chromium.org/issues/545636884

Measurement

Measure through Web Platform Test results, Interop 2026 WebTransport results, and developer feedback. No dedicated UMA metric is planned because this is an additive capability-reporting API that does not change network behavior.

Availability expectation

Expected to become available across major browser engines as part of the W3C WebTransport specification. WebKit implements both attributes, while Firefox currently implements the per-session reliability attribute

Adoption expectation

Expected to be adopted by WebTransport applications that use datagrams or need to operate across transports with different reliability capabilities.

Adoption plan

Adoption is expected to happen organically as WebTransport applications use these attributes to select between reliable streams and unreliable datagrams.

Non-OSS dependencies

Does the feature depend on any code or APIs outside the Chromium open source repository and its open-source dependencies to function?

No

Estimated milestones

Shipping on desktop	158
DevTrial on desktop	155
Shipping on Android	158
DevTrial on Android	155
Shipping on WebView	158

Anticipated spec changes

Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).

None. The attributes and  WebTransportReliabilityMode  values are already defined in the W3C WebTransport specification.

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5094058497277952?gate=5518510552907776

Links to previous Intent discussions

Intent to Prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6a9705af.b5ef4c6f.265fb1.0007.GAE%40google.com

This intent message was generated by Chrome Platform Status.

[Alex Russell]

LGTM1; thanks for closing this gap.

[Dan Clark]

LGTM2

[Mike Taylor]

LGTM3

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