PUSH_PROMISE is http/2 header that is used to add resources that are used on the page with initial request to reduce the amount of calls between server and client.
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a set of headers that provide additional context for the stream. more: https://http2.github.io/http2-spec/#PUSH_PROMISE