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HTTP/2 and HTTP/3 multiplexing: how many parallel CORS-backed fetches share one connection
Browsers coalesce requests to the same origin on a connection; preflight and data requests may be interleaved on one socket.
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http2http3cors
Latency interactions
Head-of-line blocking differs between HTTP/2 TCP and HTTP/3 QUIC; measure TTFB per resource type.
Large preflight responses can still delay streams if the server processes OPTIONS on the same thread pool.
TLS and ALPN
Negotiate h2 and h3 explicitly at the edge; clients fall back independently of your CORS policy.
0-RTT resumption on QUIC has replay implications for non-idempotent APIs—separate from CORS but plan together.