We implemented HTTP/2 server push in nghttpx and we enabled server push in nghttp2.org site.
We operate nghttp2.org site using nghttpx proxy server in front and nginx on its back. The Link header field above is generated by nginx. When nghttpx proxy sees this in response header, it initiates server push for resource denoted by path enclosed by ‘<’ and ‘>’. We specifically check link relation preload to decide the resource should be pushed or not. See the manual page to enable this feature in nghttpx.
We also push different resources, CSS and font files, under https://nghttp2.org/documentation/