...
The problem is how to decide whether to push a resource or not, because the browser may already have it cached, so the push will be a waste of bandwidth (https://http2.github.io/faq/#how-can-i-use-http2-server-push).
Articles explaining possible solutions for this problem: https://www.mnot.net/blog/2016/04/22/ideal-http, https://mariusgundersen.net/module-pusher/
Undertow 2.0.0.Alpha1 also provides implementation of PushBuilder
...