Cascading Order

Structure and Rules

Table of contents:

Rules
Declarations
Pseudo-classes and Pseudo-elements
Cascading Order

When multiple style sheets are used, the style sheets may fight over control of a particular selector. In these situations, there must be rules as to which style sheet's rule will win out. The following characteristics will determine the outcome of contradictory style sheets.

1.! important

Rules can be designated as important by specifying ! important. A style that is designated as important will win out over contradictory styles of otherwise equal weight. Likewise, since both author and reader may specify important rules, the author's rule will override the reader's in cases of importance. A sample use of the ! important statement:

BODY { background: url(bar.gif) white;
background-repeat: repeat-x ! important }

2.Origin of Rules (Author's vs. Reader's)

As was previously mentioned, both authors and readers have the ability to specify style sheets. When rules between the two conflict, the author's rules will win out over reader's rules of otherwise equal weight. Both author's and reader's style sheets override the browser's built-in style sheets.

Authors should be wary of using ! important rules since they will override any of the user's ! important rules. A user may, for example, require large font sizes or special colors due to vision problems, and such a user would likely declare certain style rules to be ! important, since some styles are vital for the user to be able to read a page. Any ! important rules will override normal rules, so authors are advised to use normal rules almost exclusively to ensure that users with special style needs are able to read the page.

3.Selector Rules: Calculating Specificity

Style sheets can also override conflicting style sheets based on their level of specificity, where a more specific style will always win out over a less specific one. It is simply a counting game to calculate the specificity of a selector.

a. Count the number of ID attributes in the selector.
b. Count the number of CLASS attributes in the selector.
c. Count the number of HTML tag names in the selector.

Finally, write the three numbers in exact order with no spaces or commas to obtain a three digit number. (Note, you may need to convert the numbers to a larger base to end up with three digits.) The final list of numbers corresponding to selectors will easily determine specificity with the higher numbers winning out over lower numbers. Following is a list of selectors sorted by specificity:

#id1 {xxx} /* a=1 b=0 c=0 --> specificity = 100 */
UL UL LI.red {xxx} /* a=0 b=1 c=3 --> specificity = 013 */
LI.red {xxx} /* a=0 b=1 c=1 --> specificity = 011 */
LI {xxx} /* a=0 b=0 c=1 --> specificity = 001 */

4.Order of Specification

To make it easy, when two rules have the same weight, the last rule specified wins.