docs(IPIP-0523): document why no HTTP 400 on format conflict

add alternative explaining why we don't error when Accept header
and ?format disagree: complexity, real-world HTTP infra behavior,
and testability concerns

Author: lidel

src/ipips/ipip-0523.md

@@ -1,6 +1,6 @@
 ---
 title: "IPIP-0523: Prefer format param over Accept header"
-date: 2025-11-26
+date: 2025-12-11
 ipip: proposal
 editors:
   - name: Alex Potsides
@@ -16,6 +16,12 @@ editors:
       url: https://ipshipyard.com
 relatedIssues:
   - https://github.com/ipfs/specs/issues/521
+thanks:
+  - name: Adin Schmahmann
+    github: aschmahmann
+    affiliation:
+      name: Shipyard
+      url: https://ipshipyard.com
 order: 523
 tags: ['ipips']
 ---
@@ -106,7 +112,9 @@ change authentication, authorization, or data integrity behaviors.
 
 ### Alternatives
 
-**Keep status quo with wildcard exception**: The previous spec already had a
+#### Keep status quo with wildcard exception
+
+The previous spec already had a
 carve-out where wildcards (e.g., `*/*`, `application/*`) in the `Accept` HTTP
 header did not take precedence over the `format` URL query parameter. This meant
 browser use cases were effectively supported, since browsers include wildcards
@@ -119,6 +127,32 @@ This alternative was rejected because:
 3. The simpler rule ("format always wins") is easier to understand and implement
 4. Real-world browser use cases work identically under both rules
 
+#### Return HTTP 400 on conflicting format preferences
+
+Require gateways to return HTTP 400 when the `Accept` header and `format`
+parameter specify different formats, signaling client misconfiguration.
+
+This was rejected as impractical:
+
+1. **Reintroduces complexity**: Detecting "conflicts" requires fully parsing
+   `Accept` headers, handling wildcards and quality values - the very complexity
+   this IPIP eliminates.
+
+2. **Incompatible with real-world HTTP infrastructure**: CDNs and reverse proxies
+   (Nginx, Cloudflare, etc.) routinely strip, transform, or ignore `Accept`
+   headers. When requests traverse multiple hops, each with different behaviors,
+   the `Accept` header often doesn't reach the gateway intact. The URL is the
+   only reliable cache key across the entire stack.
+
+3. **Untestable in practice**: Conformance tests expecting HTTP 400 would
+   spuriously pass or fail depending on whether upstream infrastructure forwarded
+   the `Accept` header. This creates a specification that cannot be reliably
+   verified.
+
+The chosen approach - `format` always takes precedence, no error on disagreement -
+is a pragmatic choice that minimizes implementation complexity and maximizes
+interoperability with real-world HTTP clients, CDNs, reverse proxies, and caches.
+
 ## Test fixtures
 
 Implementers can either write own test that prefers the `format` URL query