You might consider updating the readme to make it clear what Mercure/SSE does that WebSockets doesn't.
If it was me, I'd also put in the readme what WebSockets does that Mercure/SSE does not, such as bidirectional communication, low latency client-to-server messages, binary data transfer, etc.
Exactly this, the landing page seem much more about the reference implementation than the actual protocol. I will say the docs at https://mercure.rocks/spec are nice.
You might consider updating the readme to make it clear what Mercure/SSE does that WebSockets doesn't.
If it was me, I'd also put in the readme what WebSockets does that Mercure/SSE does not, such as bidirectional communication, low latency client-to-server messages, binary data transfer, etc.