That’s not how head-of-line blocking works. Just having a single stream doesn’t guarantee no blocking. It’s not really about unrelated requests getting in the way and sucking up bandwidth (that’s a separate issue, and arguably applies regardless of how many TCP connections you have), head-of-line blocking is about how TCP handles retransmission of lost packets. Websocket suffers from head-of-line blocking too, which is a reason that WebTransport is being developed.

Certainly, if you have other requests in flight, you could have head-of-line blocking because of a packet that was dropped in a response stream that isn’t related to your SSE stream, but this only applies if there’s packet loss, and the packets that were lost could just as easily be SSE’s or websocket’s.

I agree that HTTP/3 should solve the issue of head-of-line blocking being caused by packets lost from an unrelated stream, but it doesn’t prevent it from occurring entirely.

My understanding (which could be wrong) is that WebTransport is supposed to offer the ability to send and receive datagrams with closer to UDP-level guarantees, allowing the application to continue receiving packets even when some go missing, and then the application can decide how to handle those missing packets, such as asking for retransmission. Getting an incomplete stream at the application level is what it takes to entirely avoid head-of-line blocking.

As alluded to earlier, there is zero head-of-line blocking if there is no packet loss. Outside of congested networks or the lossy fringes of cell service, I really wonder how much of an issue this is. I’m skeptical that it adds any latency for SSE vs websocket in the average benchmark or use case. The latency should be nearly identical. Your comment seems predicated on it definitely being worse, but based on what numbers? I admit it’s been a couple of years since I measured this myself, but I came away with the conclusion that websockets are massively overrated. There are definitely a handful of use cases for websockets, but… it shouldn’t be the tool everyone reaches for.

HTTP/3 is really meant to be an improvement for a small percentage of connections, which is a huge number of people at the scale that Google operates at. I don’t think there are really any big downsides to HTTP/3, so I look forward to seeing it mature, become more common, and become easier to find production grade libraries for.