A CSS declaration block is an ordered collection of CSS properties and values. It is represented in the DOM as a CSSStyleDeclaration.

Each property and value pairing is known as a CSS declaration. The CSS declaration block has the following associated properties:

computed flag

Set if the CSSStyleDeclaration object is a computed rather than specified style. Unset by default.

declarations

The CSS declarations associated with this object.

parent CSS rule

The CSSRule that the CSS declaration block is associated with, otherwise null.

owner node

The element that the CSS declaration block is associated with, otherwise null.

updating flag

Set when the CSS declaration block is updating the owner node's style attribute.

When a CSSStyleDeclaration is returned by a CSS Object Model (CSSOM) interface these properties are set to appropriate values as defined by the specification.

The following example shows a CSS rule with a declaration block for the h1 element. The CSS declaration block is the lines between the curly braces.

h1 {
  margin: 0 auto;
  font-family: "Helvetica Neue", "Arial", sans-serif;
  font-style: italic;
  color: rebeccapurple;
}

We can return a CSSStyleDeclaration representing this CSS declaration block using CSSStyleRule.style.

let myRules = document.styleSheets[0].cssRules;
let rule = myRules[0]; // a CSSStyleRule
console.log(rule.style); // a CSSStyleDeclaration object