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