MSC4138: Update allowed HTTP methods in CORS responses (#4138)

proposals/4138-update-cors-methods.md

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+# MSC4138: Update allowed HTTP methods in CORS responses
+
+The [specification](https://spec.matrix.org/v1.10/client-server-api/#web-browser-clients) suggests
+that servers allow a limited subset of the available [HTTP methods](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods)
+available in [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) responses. However, it's
+reasonable to expect the specification to use other methods in the future or as part of feature
+detection. To permit these use cases early, this MSC proposes adding a few more allowable values to
+the `Access-Control-Allow-Methods` header.
+
+## Proposal
+
+The [`Access-Control-Allow-Methods` header's recommended value](https://spec.matrix.org/v1.10/client-server-api/#web-browser-clients)
+is updated to include the following:
+
+* `PATCH` - A plausibly useful HTTP method for future use.
+* `HEAD` - Similar to `PATCH`, `HEAD` is plausibly useful for feature detection and cases like
+  [MSC4120](https://github.com/matrix-org/matrix-spec-proposals/pull/4120).
+
+The following methods are *not* included because they don't have foreseeable use in Matrix:
+
+* `CONNECT`
+* `TRACE`
+
+## Potential issues
+
+None anticipated.
+
+## Alternatives
+
+No significant alternatives.
+
+## Security considerations
+
+CORS is meant to help ensure requests made by the client are properly scoped in the client. If the
+client wishes to use an HTTP method not allowed by the server, the web browser will mask the
+response with an error before the application can inspect it. Therefore, to increase future
+compatibility, we append a few useful HTTP methods while still excluding ones which are (currently)
+nonsensical.
+
+## Unstable prefix
+
+This proposal cannot have an unstable prefix due to the nature of CORS. Servers are already able to
+go off-spec and serve different headers because the spec is merely a recommendation.
+
+## Dependencies
+
+This proposal has no dependencies.