[0x0]

What if the browser already has those in the client cache? Will it have to abort the pushes, and will it even be able to do so in time on a high bandwidth high latency network like 3g/4g?

Is there a risk that cellular data usage will increase from this?

[dcsommer]

Yes there is. Knowing when a client has a resource cached already is an important part of server push. There is indeed a risk of over pushing.