The cascade is an algorithm that defines how user agents combine property values originating from different sources. The cascade defines the origin and layer that takes precedence when declarations in more than one origin or cascade layer set a value for a property on an element.

The cascade lies at the core of CSS, as emphasized by the name: Cascading Style Sheets. When a selector matches an element, the property value from the origin with the highest precedence gets applied, even if the selector from a lower precedence origin or layer has greater specificity.

This article explains what the cascade is and the order in which CSS declarations cascade, covering cascade layers and origin type. Understanding origin precedence is key to understanding the cascade.