> I can't see a way that new protocols are going to solve this, unless I'm missing something in how HTTP2 is going to be handled by the browser?

HTTP/2 server push. Your server can proactively deliver things like rollover state graphics knowing that the client will need them.

good point. I imagine determining when to push these assets will become a complex choice though.

Also, I realized shortly after commenting that I missed the obvious benefit of avoiding downloading global spritesheets and other compiled assets for individual pages that only use a subset of the content.

Spritesheets and other forms of asset concatenation are aiming to reduce the impact of round-trip latency and overhead of HTTP i.e repeated headers.

I've always seen the "image is already loaded" as a "nice" side effect but spritesheets can have issues in mobile contexts as the whole image must be decoded jsut to access the sprite, it's unclear how effectively browsers cache the individual sprites in memory, compared to individual images too.

I would consider continuing to spritesheet the button itself. The main difference is that each roll-over effect can be separated from all the other spritesheets in the world.

Still, an exciting time when we can combine files based on what is the most logical grouping, rather than what is the most efficient. I look forward to the day when HTTP2 rules the world.