Can diagrams be "built" offline?

[xeruf]

From the README I am still not quite clear whether simple diagrams can be built offline or whether it always requires the website?

[josteink]

It depends 100% on the online web service to actually render the diagrams.

You are free to edit the sequence diagram code while offline though.

If you need offline-capabilities, I would suggest using PlantUML instead, which also has a corresponding Emacs major-mode. The main con is that it's a bit more involved to setup, but it's nowhere near impossible.