FrontendHaltNetworking·Advanced·Conceptual·1 min read
What is head-of-line blocking, and how do HTTP/2 and HTTP/3 differ on it?
Short interview answer
It's when one delayed item blocks everything queued behind it. HTTP/2 removed it at the HTTP layer but all streams still ride one TCP connection, so a single lost packet stalls every stream until it's retransmitted. HTTP/3 runs over QUIC, where each stream's loss recovery is independent, so one lost packet only affects its own stream.
Key takeaway
Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.