The main feature that is required is the ability to callcc and/or coroutine yield from any point in the stack. If you look at Wee, the user of the framework has to callcc all over the place to make it work. In Tir I have web:prompt(), which itself does the yielding without anyone having to care.

The significance of this is that Tir can provide one API for all three styles without special effort from the user.