Yes, they are generated offline. It's the most time consuming step when generating the terrain.

Doing the lighting and shadow generation in real-time (during tile initialization) is a very interesting problem! The great advantage is the bandwidth reduction and the ability to move the sun of course. A Web Worker can (probably should) be used for that purpose. The problem is that in order to correctly light a tile, you need access to its adjacent tiles as well (since peaks near the border on their side may be casting shadows on our tile).

All in all, very interesting and certainly doable!