Create MSC with fixes for client well-known

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Author: aaronraimist

proposals/xxxx-client-well-known-fixes.md

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+# MSCXXXX: Fixes for Client Well-known URI
+
+Several minor issues and inconsistencies have cropped up since the `/.well-known/matrix/client`
+method was added to allow clients to discover servers. This proposal intends to fix these issues.
+
+## Proposal
+
+The following changes should be made to the client server spec:
+
+1. The spec does not mention that redirects should be followed. The spec should be changed to
+state that when a client is looking up the well-known URI, it should follow HTTP 3xx redirects
+up to 30 times. This is consistent with the redirects followed when a server resolves a
+server name using `/.well-known/matrix/server`.
+
+1. Currently the spec does not mention which Content-Type should be used for the response to
+`/.well-known/matrix/client`. The Content-Type SHOULD be `application/json` however it
+should be assumed to be JSON regardless of type. This is consistent with the Server-Server API.
+
+1. When a client is following the flow described in the spec for looking up `/.well-known/matrix/client`,
+the spec currently states, in part:
+
+    > 3․ Make a GET request to https://hostname/.well-known/matrix/client.<br>
+	>>  a․ If the returned status code is 404, then `IGNORE`.<br>
+    >>  b․ If the returned status code is not 200, or the response body is empty, then `FAIL_PROMPT`.
+
+    In practice most web servers do not add CORS headers on 404 errors by default. Therefore
+    web based clients cannot always determine if the status code is 404. Step 3b should be 
+    changed from `FAIL_PROMPT` to `IGNORE` so that a non 200 response is treated in the same
+    way as 404. This change is intended to fix issues like https://github.com/vector-im/riot-web/issues/7875.
+  
+    This change does have potential security concerns, see https://github.com/vector-im/riot-web/issues/11136.
+
+1. Step 3f in the flow should be changed to use the modern `/_matrix/identity/v2` API to
+validate the identity server rather than the deprecated `/_matrix/identity/api/v1`. Clients
+should fall back to the v1 API as described in the identity service spec.