Style sheets would, particularly in concert with compression, for our Microscape test:
Reduce the number of bytes by >50% (with corresponding improvement on dialup lines)
Halve the number of URL's looked up by the server (increasing server performance by a corresponding amount)
Other content changes coming:
PNG deployment, vector graphics
For dialup users, in our test, style sheets would do more for observed performance to most people in the internet than everything we do in the protocol!