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Can I use it for SPA? Something like an Angular app with all the code running on the client side with routing (no SSR).

Not currently, but this will be implemented soon (and certainly before 1.0) https://github.com/sveltejs/kit/issues/231

Will SvelteKit follow the same pattern as Sapper?

As in, after the first SSR'd page the app becomes an SPA.

Yes

Hopefully, it will be possible to disable this behavior.