CSS Cascading and Specificity Cascading


Example

Cascading and specificity are used together to determine the final value of a CSS styling property. They also define the mechanisms for resolving conflicts in CSS rule sets.

CSS Loading order

Styles are read from the following sources, in this order:

  1. User Agent stylesheet (The styles supplied by the browser vendor)
  2. User stylesheet (The additional styling a user has set on his/her browser)
  3. Author stylesheet (Author here means the creator of the webpage/website)
    • Maybe one or more .css files
    • In the <style> element of the HTML document
  4. Inline styles (In the style attribute on an HTML element)

The browser will lookup the corresponding style(s) when rendering an element.

How are conflicts resolved?

When only one CSS rule set is trying to set a style for an element, then there is no conflict, and that rule set is used.

When multiple rule sets are found with conflicting settings, first the Specificty rules, and then the Cascading rules are used to determine what style to use.

Example 1 - Specificity rules

.mystyle { color: blue; }  /* specificity: 0, 0, 1, 0 */
div { color: red; }        /* specificity: 0, 0, 0, 1 */
<div class="mystyle">Hello World</div>

What color will the text be? (hover to see the answer)

blue

First the specificity rules are applied, and the one with the highest specificity "wins".

Example 2 - Cascade rules with identical selectors

External css file

.class {
  background: #FFF;
}

Internal css (in HTML file)

<style>
.class {
  background: #000;
}
<style>

In this case, where you have identical selectors, the cascade kicks in, and determines that the last one loaded "wins".

Example 3 - Cascade rules after Specificity rules

body > .mystyle { background-color: blue; }   /* specificity: 0, 0, 1, 1 */
.otherstyle > div { background-color: red; }  /* specificity: 0, 0, 1, 1 */
<body class="otherstyle">
  <div class="mystyle">Hello World</div>
</body>

What color will the background be?

red

After applying the specificity rules, there's still a conflict between blue and red, so the cascading rules are applied on top of the specificity rules. Cascading looks at the load order of the rules, whether inside the same .css file or in the collection of style sources. The last one loaded overrides any earlier ones. In this case, the .otherstyle > div rule "wins".

A final note

  • Selector specificity always take precedence.
  • Stylesheet order break ties.
  • Inline styles trump everything.