Let's step into our time machine for a second
It's 1964 and you just received this Memorandum For Members and Affiliates of the Intergalactic Computer Network from one of your colleagues, a fellow named Paul Baran.
This memo describes how distributed networks are more reliable than centralized networks, meaning distributed networks are harder to destroy. Why is that the case? because in a distributed network, "complete reliance upon a single point is not always required". In other words, distributed networks seek to avoid a single point of failure.
The big idea
You can guess what this intergalactic computer network is called today. The whole point of the internet was to create a digital communications network that avoids a single point of failure. That is the foundation that the internet is built on.
A new approach
This is what I think progressive enhancement should mean in web-development today: Does your website try to avoid a single point of failure?