Server Push allows you to push a set of files down to the browser along with the request for the HTML even before the HTML is parsed. This means sending things like logos and CSS before the browser knows to request them, and thus speeding up the perceived load time of your page.
Let’s say that you had a site that runs on PHP, which of course your WordPress instance does, and you wanted to use HTTP2 Server Push to push down two CSS files and your logo.
You can use the following code which will do just that for you…
來源： Configuring HTTP2 Push with WordPress | Responsive Web Design