Monitoring/alarming, and knowing what to monitor. Also, properly instrument your services or whatever it is you have. Take time to reflect on what are the signals that tell you operational health. An error metric alone is useless if you don’t know the denominator. Also be careful to avoid adding noisy metrics that cause panic for no reason.
I’m not sure what fault tolerance means in this context. Very handwavy statement. I think if you have dependencies, have a plan and understanding of which ones tipping over will bring down your service or how you can build resiliency. For example, some feature on your page requires talking to a recommendations service. If the service goes down, can you call back to a generic list of hard coded recommendations or some static asset?
As for automation: yeah, have test workflows built into your CI/CD harness. And avoid manual steps there requiring human intervention. Use canaries to test certain functions are up and running as expected, etc