Web Design in a Nutshell, 3rd Edition

Conflicting Style Rules: The Cascade

It is possible (even common) for elements in a document to get presentation instructions from several sources. Conflicts are certain to arise. The working group that developed CSS anticipated this situation and devised a hierarchical system that assigns different weights to various sources’ style information. The cascade (of Cascading Style Sheets) refers to what happens when several sources of style information vie for control of the elements on a page; style information is passed down until it is overridden by a style command with more weight.

The cascade order provides a set of rules for resolving conflicts between competing style sheets. When a user agent (such as a browser) encounters an element, it looks at all of the style declarations that might possibly apply to it, and then sorts them all out according to style sheet origin, selector specificity, and rule order to determine which one applies.