They mention this in the doc, but it seems like the future solution to preloading resources will involve HTTP Status 403 [1]. That seems like a great alternative to server push.

Currently it seems like the best option is to use Transfer-Encoding: chunked and make sure you flush out an HTML header that includes the preload directives before you spend any time rendering the web page. This is a pain - especially if something in the rendering fails and you now have to figure out how to deliver...something to the client. It also limits other headers significantly - e.g. cookies.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/103