SupportsCSS
Live, in-browser detection of modern CSS support for selectors, features, and at-rules. Applies support-based classes, exposes a results object, and allows custom tests.
Inspired by the legacy of Modernizr, this script evaluates a user's browser for cutting-edge modern CSS support beyond the capabilities of @supports.
- Classes are added to
<html>as eithersupports-[feature]orno-[feature], allowing easier progressive enhancement and build strategies - Checks for selectors like
:has(), properties liketext-box-trim, features like relative color syntax, and at-rules like@layer- see full test suite - Allows adding custom tests
- Exposes a results object to iterate over detected support, as well as individual results for quick conditional checks in JS
SupportsCSS is not a polyfill, it is only feature detection. Continue using tools like PostCSS or LightningCSS for prefixing and other features like syntax lowering. SupportsCSS is a layer on top of those tools.
Installation & Usage
Review the full docs at SupportsCSS.dev.
Test Suite
| Feature Class | Global Name | Test Condition |
|---|---|---|
| at-container | AtContainer | window.CSSContainerRule |
| at-container-style-properties | AtContainerStyleProperties | * See explanation |
| at-counter-style | AtCounterStyle | window.CSSCounterStyleRule |
| at-layer | AtLayer | window.CSSLayerBlockRule |
| at-property | AtProperty | window.CSSPropertyRule |
| at-scope | AtScope | window.CSSScopeRule |
| at-starting-style | AtStartingStyle | window.CSSStartingStyleRule |
| anchor-positioning | AnchorPositioning | CSS.supports('anchor-name: --a') |
| color-function | ColorFunction | CSS.supports('color: color(srgb 0 0 1)') |
| color-mix | ColorMix | CSS.supports('color: color-mix(in lch, white, black)') |
| container-units | ContainerUnits | CSS.supports('width: 1cqi') |
| dynamic-viewport-units | DynamicViewportUnits | CSS.supports('width: 1dvi') |
| has | Has | CSS.supports('selector(:has(+ *))') (Possible false positive in Firefox 112) |
| houdini-paint-api | HoudiniPaintApi | window.CSS.paintWorklet |
| individual-transforms | IndividualTransforms | CSS.supports('transform: scale(1)') |
| light-dark | LightDark | CSS.supports('color: light-dark(black, white)') |
| logical-properties | LogicalProperties | CSS.supports('border-start-start-radius: 1px') |
| media-range-syntax | MediaRangeSyntax | window.matchMedia('(width >= 1px)') |
| nesting | Nesting | CSS.supports('selector(& a)') |
| nth-of-s | NthOfS | CSS.supports('selector(:nth-child(1 of .a))') |
| overscroll-behavior | OverscrollBehavior | CSS.supports('overscroll-behavior: none') |
| relative-color-syntax | RelativeColorSyntax | CSS.supports('color: rgb(from red r g b / 1%)') |
| scroll-timeline | ScrollTimeline | CSS.supports('scroll-timeline-name: --a') |
| subgrid | Subgrid | CSS.supports('grid-template-rows: subgrid') |
| text-box-trim | TextBoxTrim | CSS.supports('text-box-trim: both') |
| trigonometry | Trigonometry | CSS.supports('width: calc(1px * cos(1deg))') |
| user-invalid | UserInvalid | CSS.supports('selector(:user-invalid)') |
| user-valid | UserValid | CSS.supports('selector(:user-valid)') |
| view-timeline | ViewTimeline | CSS.supports('view-timeline-name: --a'), |
| view-transitions | ViewTransitions | window.ViewTransition |
How were these features selected?
Features were selected based on:
@supportslimitations- instability of the spec
- freshness to the language
- impact on CSS architecture
- impact on progressive enhancement
Colophon
Created by Stephanie Eckles, author of ModernCSS.dev, SmolCSS.dev, and other front-end dev resources.