+1, SSE is great. We (Twitch) use it between systems in a few key cases since it is really easy to make a client for it if you find yourself in a language that doesn't already have it baked into their HTTP client offering.

When it comes to browsers we've still not found a solution that is as solid as Flash sockets; though that could also be a bit of a hangover for us given our reliance on Flash and the subsequent tooling that we have around it.

Here's a polyfill: https://github.com/Yaffle/EventSource/

2 issues; Last-Event-ID is not a header, but a querystring param. Needs 2kb of padding from server before any data.

Looks pretty usable.

SSE is listed as "under construction" on the IE roadmap: http://status.modern.ie/serversenteventseventsource

They are very easy to shim with polling though.

MS tried to look all hip and cool with IE10/11 and SSE didn't have much buzz around it so it wasn't implemented.