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It's a hard problem because there are many different things which can cause downtime: S3 is simple and redundant but also limited so they constrain the engineering problem to something manageable.

If you need anything more complex than delivering static content, you have to spend the effort thinking about what's important to you and how you're comfortable spending and/or compromising to meet that reliability goal. The answers for this tend to be pretty specific to your business needs and tech stack.

Data synchronization between data centers is tough. There's A LOT of data moving in those data centers all once--it isn't easy to mirror that across the country in real time.

That's why the good lord invented eventually consistent data synchronization models. If your use case can tolerate it, use it.

There is an off the shelf solution - Google App Engine.