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Style Sets

Style set is a named set of style declarations – it is a system of selectors and style definitions that are applied as a whole.

Essentially style sets are scoped style modules.

Style Set Declaration

Style set declaration is an at-rule with syntax similar to @media sections:

@set name [< parent-set-name] {
/* content rules: */
:root { ... }
:root > child { ... }
child { ... }

Element that has style set applied (hosts the set) is a root of the set - can be selected in style set using :root selector.

Rules inside SS can be applied only to the host element and its children.


Style sets support inheritance using @set name < parent-set-name {...} declaration form. In this case new set with the name initially gets all rules from parent-set-name set and its own rules are getting appended to the list.


Rules inside the set use normal CSS specificity calculation. More specific rules applied after less specific. Inherited rules and self defined rules are not different in this respect - more specific inherited rule may override less specific self defined rule.

Style Set Application

Sets can be applied to host elements these ways:

  • by CSS
  • by HTML
  • by JS/JSX

CSS Application

Style set can be attached to particular elements using style-set property in CSS:

selector {
style-set: set-name [url(file.css)];

If url(file.css) is provided it gets loaded on demand and a set is looked up by set-name there.

HTML Application

Style sets can be applied to elements using styleset attribute:

<div styleset="styles.css#set-name">
... content styled by SS ...

The styleset accepts url of CSS file where the set is defined and an anchor (after #) that is a name of that @set in that file.

JS/JSX Application

Style sets can be applied in JSX templates using the same styleset attribute:

render() {
return <div styleset={__DIR__ + "styles.css#set-name"}>

__DIR__ + ... prefix above is required to make absolute url of that CSS file. It combines url of this .js file with the "styles.css#set-name" relative url.

JS/JSX resident CSS declarations

Style sets can be defined locally in js files instead of external CSS files. You may want such local style definitions when designing single JS file based components.

JS/JSX resident style set are declared by using built-in CSS.set() function. That function supports tagged template usage:

const styleSet = CSS.set`
:root { ... }
:root > child { ... }
child { ... }

This defines anonymous style set - no name and { } brackets are used. The style set gets compiled and stored in the styleSet (or any other) JS variable.

Such code resident style set can be applied by using the same styleset attribute:

render() {
return <div styleset={styleSet}>

Overriding Style Set Rules from Outside

Rules of style sets are applied on top other rules. To override properties defined in style sets use !important in normal style rules:

@set TestSet {
:root {

div {
style-set: TestSet;
color:green !important; /* #1 */
background:red; /* #2 */

Rule #1 will change the color and #2 will NOT change the background here:

<div>This should be green text on gold background</div>