The thought behind this is escalators provide great functionality. They move you up and down, what a great feature! But what if they break? Well, then they're just stairs. They don't have that neat feature, but their core function is still there, to get you up and down floors.
Imagine if this was how escalators worked. While they're powered, they give you that great functionality of moving you up and down. But an escalator that worked like a web page, when it lost power and didn't have the extra feature, instead of stairs the steps would slope down becoming a slide you couldn't walk up.
I don't know if this concept was already a thing (again, not a web developer), but I'd like to see it more.