LGTM3
On 10/12/26 2:41 p.m., 'Dan Clark' via blink-dev wrote:
LGTM2
On Monday, October 12, 2026 at 11:40:41 AM UTC-7 [email protected]
wrote:
LGTM1; thanks for closing this gap.
On Wednesday, October 7, 2026 at 10:23:27 AM UTC-7 Chromestatus wrote:
*Contact emails*
[email protected]
*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
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3ENetwork%3EWebTransport%22>
*Web Feature ID*
webtransport <https://webstatus.dev/features/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
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?*
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
<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
<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
<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
<https://chromestatus.com>.
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