None, the web has always been (or at least intended to be) backwards-compatible, if you disable concatenation (for ex.), your HTTP1 users will suffer.
OTOH, techniques like ES6 modules won't take advantage of HTTP2 if you implement them incorrectly, there is a reason why they aren't supported by browsers yet.
K
Ha!
Vijay Thirugnanam
Inference Service @ Cerebras
If server push becomes mainstream, calling web API to retrieve data becomes redundant. Web will be modelled similar to socket.io and Meteor publish / subscribe.