Any web-based stuff, I find the easiest is for both parties to simply work off a single server. This means:
- Either taking turns in editing the file on the server, and pressing "reload" in your browser, so that both of you see the results at the same time.
- Or both sides working against a version control repository (Subversion, or Git like was mentioned here already) and keep updating that on the server.
This requires a server of some kind with those abilities, but since anyone can install something like that on their own computer, or just pay a few dollars a month for someone else to host it for him, it doesn't seem like such a big deal.
