These things aren't magical. You will have some downtime because your connection to the database will be interrupted (after all, it just died, no way around it), and depending on your configuration, you may lose some data; you need at least one synchronous replica to ensure that no data is lost on an uncontrolled failover. If you have a two-node "cluster" with synchronous replication, then either one going down will stop writes on the master, so you would need at least three nodes.

Mind you, no clustered system can provide you zero downtime and zero data loss without essentially the same constraints. If you have an unplanned failure, there's going to be a blip somewhere. It's up to the rest of the system to recover from it gracefully.

When configuring any kind of cluster involving data (especially one that's supposed to recover automatically from faults), make sure you understand how it behaves and adjust for your application, because quite often the default configurations might accept some data loss in unplanned failover or conflict scenarios.

will probably lose some data but easy enough to test with this thing