Merge pull request #575 from jan-ivar/featuredetect2

Add WebTransport.supportsReliableOnly boolean.

index.bs

@@ -622,6 +622,8 @@ interface WebTransport {
   /* a ReadableStream of WebTransportReceiveStream objects */
   readonly attribute ReadableStream incomingUnidirectionalStreams;
   WebTransportSendGroup createSendGroup();
+
+  static readonly attribute boolean supportsReliableOnly;
 };
 
 enum WebTransportReliabilityMode {
@@ -948,6 +950,10 @@ these steps.
    requested but not satisfied, then the value is `"default"`
    The getter steps are to return [=this=]'s {{[[CongestionControl]]}}.
 
+: <dfn for="WebTransport" attribute>supportsReliableOnly</dfn>
+:: Returns true if the user agent supports [=WebTransport sessions=] over exclusively reliable
+   [=connections=], otherwise false.
+
 ## Methods ##  {#webtransport-methods}
 
 <div algorithm>