Perhaps even sales of non-tangible products delivered electronically vs. diskettes. US Federal taxation is an even more complicated question. As for now there are no rules because of decisions that past rules should apply to the new era. There is an emphasis on residence based tax. Cyberspace location is not treated as a permanent location.
State taxation is also complicated.
Most businesses do not keep a record of what customers are browsing on a daily basis. Many states charge no taxes for electronically distributed software. Other countries show very different strategies. It is unclear whether a server means permanent establishment. In Germany, physical equipment constitutes a permanent establishment.