CSS variables are entities defined by CSS authors that contain specific values to be reused throughout a document. They are set using custom property notation (e.g., --main-color: black;) and are accessed using the var() function (e.g., color: var(--main-color);).

Complex websites have very large amounts of CSS, often with a lot of repeated values. For example, the same color might be used in hundreds of different places, requiring global search and replace if that color needs to change. CSS variables allow a value to be stored in one place, then referenced in multiple other places. An additional benefit is semantic identifiers. For example, --main-text-color is easier to understand than #00ff00, especially if this same color is also used in other contexts.

CSS variables are subject to the cascade and inherit their value from their parent.