With HTTP/2 Push, can I push assets before the initial response?

I have a webpage that has JS, CSS, and font assets. The page has to do some heavy processing before determining the HTTP Status code and headers for its response.

I'd like to use HTTP/2 Push to send the assets to the browser without waiting on this heavy processing. The timeline would look something like this:

1. Client requests index.html
2. Server sends PUSH_PROMISES for script.js, styles.css, and font.woff2
3. Server sends HTTP headers and data for the assets above
4. Server does some heavy processing to determine index.html response...
5. Server sends HTTP headers and data for index.html

Is this possible? Based on my understanding of Server Push in the HTTP/2 spec, it appears possible. However, it's my first time diving into the HTTP/2 spec, so I could definitely be missing something.

Yes that’s totally allowed and it’s possible in Apache for instance as detailed here: https://icing.github.io/mod_h2/earlier.html

Or an example with Node is given here: https://github.com/bazzadp/http2-in-action/blob/master/Listing%205.3/app.js

Other servers may allow this too, but many servers use preload HTTP headers as signals to push so that needs the response to be sent sent back to show the header.

An extra 103 Early Hint response has been defined which can be sent back early with these headers while your main response is being processed, however support of this is poor not least because some implementations will be confused to get back two responses (a 103 followed by a 200).