[ecesena]

My personal feeling is that signup of a new user is too critical of a function to depend upon an external service. I would very much prefer a client-side library as others have stated.

This said, I can see a ton of potential for a plug & play solution, for example (but not limited to) a signup form for wordpress, a react input password component with a strength bar, etc.

If you create "a product that people love", I don't think it does really matter whether you verify on the client side, or securely via a remote service. You may explain pros and cons, and let the admin decide.

But I'd focus on the final solution, rather than just the service itself.