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Server-sent-events use this header with CORS http://www.w3.org/TR/eventsource/#last-event-id
More importantly, it's not an author request header.
@Anne, LOL!!! thank you for your understanding you just stop polyfilling Server sent events with XMLHttpRequest
Ah yes, that would indeed not work without a preflight request. Is that a big problem?
Also, at this time it seems easier to get browsers to add EventSource support than to change their CORS implementations yet again.
+ Last-Event-ID can be setted with help of EventSource, than disconnect + redirect can be done to make request with "Last-Event-ID" header to some other site.
or may be it is time to drop Server sent events... and move to WebSockets
I don't understand comment 5. WebSocket is great, but quite different from EventSource and EventSource is not going away at this point.
comment #5: EventSource allows to make requests with "Last-Event-ID" header without preflight.
https://github.com/mozilla/releases-mozilla-central/blob/master/content/base/src/nsXMLHttpRequest.cpp#L3037
That seems like a bug in Gecko to me. We also instruct the browser to set Referer and Origin to certain values. They're not simple either.
*** Bug 19315 has been marked as a duplicate of this bug. ***