Why do they call them cascading sheets anyway?

A cascade is defined as “a chemical or physiological process that occurs in successive stages, each of which is dependent on the preceding one, and often producing a cumulative effect”

So how do they cascade?

Well firstly one page can have heaps of em. They can be external, in the head of the html and in the html element itself (called inline).

So how do they become one?

They have a priority order. Starting with the external ones they can be overidden by the ones in the head which can be overidden by the inline.

External

Head

Inline