Server Sent Events

First attempt at using default mitmproxy with my local site failed, SSE seems not to come through. Is there something I should set somewhere?

This testpage should show updates on wikipedia articles, but is just waiting for connection.

<!DOCTYPE html>
    <title>SSE test</title>
    <div id="stream"></div>
      var source = new EventSource('');
      source.onmessage = function(event) {
        document.getElementById("stream").innerHTML += + "<br>";