Cascade

The cascade is a fundamental feature of CSS. It is an algorithm defining how to combine properties values originating from different sources. It lies at the core of CSS as stressed by its name: Cascading Style Sheets.

What CSS entities participate in the cascade

Only CSS declarations, that is property/value pairs, participate in the cascade. That means that at-rules containing entities other than declarations, like @font-face containing descriptors don't participate in the cascade. In these case, only the at-rule as the whole participates in the cascade; here the @font-face identified by its font-family descriptor. If several @font-face with the same descriptor are defined, only the most adequate @font-face, as a whole, is considered.

If the declarations contained in most at-rules participate in the cascade, like declarations contained in @media, @documents, or @supports, declarations contained in @keyframes doesn't. Like for @font-face, only the at-rule as a whole is selected via the cascade algorithm.

Finally note that @import and @charset obey specific algorithms and aren't affected by the cascade algorithm.

Origin of CSS declarations

The CSS cascade algorithm wants to select CSS declarations to set the correct value for CSS properties. CSS declarations originate from different origins:

The browser has a basic style sheet that gives a default style to any document. These style sheets are named user-agent stylesheets. Some browsers uses actual style sheets to perform this, while others simulate them in code, but both cases should be indetectable. Some browsers also allow users to modify the user-agent stylesheet. Although some constraints on user-agent stylesheets are set by the HTML specification, browsers still have a lot of latitude: that means that significant differences exist from one browser to another. To ease their development and lower the basic ground on which to work, Web developers often use a CSS reset style sheet, forcing common properties values to a known state.
The author of the Web page define styles for the document. These are the most common style sheets. Most of the time several of them are defined and they make the look and feel of the Web site — its theme.
The reader, the user of the browser, may have a custom style sheet to tailor its experience.

Though style sheets come from these different origins, they overlap in scope: the cascade algorithm defines how they interact.

Cascading order

The cascading algorithm determines how to find the value to apply for each property for each document element.

It first filters all the rules from the different sources to keep only the rules that apply to a given element. That means rules whose selector matches the given element and which are part of an appropriate media at-rule.

Then it sorts these rules according to their importance, that is, whether or not they are followed by !important, and by their origin. The cascade is in ascending order, which means that !important values from a user-defined style sheet have precedence over normal values originated from a user-agent style sheet:

	Origin	Importance
1	user agent	normal
2	user agent	`!important`
3	user	normal
4	author	normal
5	CSS Animations	see below
6	author	`!important`
7	user	`!important`

In case of equality, the specificity of a value is considered to choose one or the other.

CSS animations and the cascade

CSS Animations, using @keyframes at-rules defines animations between states. Keyframes don't cascade, meaning that at any time CSS takes values from one single @keyframes and never mixes several of them.

When several keyframes are appropriate, it chooses the latest defined in the most important document, but never combined all together.

Also note that values within @keyframes at-rules overwrite all normal values but are overwritten by !important values.

Example

User-agent CSS:

li { margin-left: 10px }

Author CSS 1:

li { margin-left: 0 } /* This is a reset */

Author CSS 2:

@media screen {
  li { margin-left: 3px }
}

@media print {
  li { margin-left: 1px }
}

User CSS:

.specific { margin-left: 1em }

HTML:

<ul>
  <li class="specific">1<sup>st</sup></li>
  <li>2<sup>nd</sup></li>
</ul>

In this case, declarations inside li and .specific rules should apply. No declaration is marked as !important so the precedence order is author style sheets before user style sheets or user-agent stylesheet.

So three declarations are in competition:

margin-left: 0

margin-left: 3px

margin-left: 1px

The last one is ignored (on a screen), and the two first have the same selector, hence the same specificity: it is the last one that is then selected:

margin-left: 3px

Note that the declaration defined in the user CSS, though having a greater specifity, is not chosen as the cascade algorithm is applied before the specifity algorithm.