Use CSS feature detection to apply an enhancement only when a browser recognizes the exact CSS capability it needs. In CSS, write a usable baseline first, then place the enhanced rules inside @supports. This approach—often called progressive enhancement—adapts to capabilities rather than guessing from a browser’s name. The key limitation: a passing query shows that the browser considers the tested syntax valid; it does not prove the feature works correctly or completely.
Write a baseline, then enhance it with @supports
CSS feature queries let you conditionally apply styles according to whether a browser accepts a declaration or other supported condition. Start with styles that keep the content usable, then add the enhancement behind a query for the capability it requires.
/* Baseline: cards remain in a simple vertical flow. */
.cards {
display: block;
}
.cards > * + * {
margin-block-start: 1rem;
}
/* Enhancement: use a grid when the browser accepts it. */
@supports (display: grid) {
.cards {
display: grid;
grid-template-columns: repeat(3, minmax(0, 1fr));
gap: 1rem;
}
.cards > * + * {
margin-block-start: 0;
}
}Outside the query, the baseline remains in effect. Inside it, the grid rules replace the baseline layout when the browser accepts display: grid. The adjacent-sibling margin is also removed in the enhanced layout so the fallback spacing does not combine with the grid gap.
Test the condition that the enhancement actually depends on. Support for one related property is not proof that a browser supports every value, selector, or behavior your design uses.
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Combine conditions and test selectors
Use and, or, and not to express more specific conditions. Parentheses make each declaration test explicit:
@supports (display: grid) and (gap: 1rem) {
.layout {
display: grid;
gap: 1rem;
}
}
@supports (display: grid) or (display: flex) {
.layout {
/* Styles suitable for either supported layout mode. */
}
}
@supports not (display: grid) {
.layout {
/* Optional targeted alternative for browsers without grid support. */
}
}A not branch is useful when a specific alternative is needed. It is not mandatory: if your ordinary CSS already provides a suitable fallback, avoid duplicating rules in a separate negative branch.
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Feature queries can also test whether a selector is supported. For example, this checks whether the browser accepts the :has() selector syntax:
@supports selector(:has(a)) {
.card:has(a) {
outline: 2px solid currentColor;
}
}As with declaration checks, selector acceptance is not a guarantee that every behavior you care about is free of implementation bugs.
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Use CSS.supports() only when JavaScript needs the answer
If JavaScript must choose a stylesheet or activate code based on CSS capability, use the browser’s CSS.supports() API. It accepts either a property and value or a full supports-condition string, and returns a boolean.
if (CSS.supports("grid-template-columns", "subgrid")) {
// Load or activate behavior that depends on subgrid.
}You can also pass a condition string:
if (CSS.supports("(display: grid) and (gap: 1rem)")) {
// Make a JavaScript decision based on this CSS condition.
}For styling alone, prefer @supports in the stylesheet. It keeps presentation logic in CSS and avoids using JavaScript to solve a CSS-only problem.
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Choose the right compatibility check
| Question | Use | What it tells you |
|---|---|---|
| Should these styles apply when a browser accepts a CSS capability? | @supports |
Whether the browser considers the tested declaration or condition valid. |
| Does JavaScript need to branch on CSS support? | CSS.supports() |
A boolean result for a property/value pair or supports condition. |
| Which browser versions report support for this exact feature? | Compatibility information for the feature | Reported support by browser and version; check the specific feature rather than a neighboring one. |
| Does the feature render or behave correctly in the environments that matter? | Testing in target browsers and devices | The actual result in the environments you choose to test. |
These checks answer different questions. A support query is a capability test, not a browser-version lookup or a rendering test. Compatibility information helps you decide which environments to target, while browser testing can reveal visual differences, incomplete implementations, and bugs.
Verify the result in target browsers
- Define the baseline. Make sure the content and essential interaction remain usable without the enhancement.
- Identify the exact dependency. List the property, value, selector, or combination the enhanced design requires.
- Write the condition narrowly. Use a declaration or selector query that tests that dependency, rather than inferring support from browser identity.
- Check feature compatibility. Consult current compatibility information for the exact feature and relevant browser versions.
- Test actual behavior. Run the page in the browsers and devices relevant to your users and inspect both the baseline and enhanced result. Cross-browser test tooling can run tests against multiple browser engines, but results still depend on the environments and cases you exercise.
Common problems and fixes
- The enhanced rules never apply. Check the property/value spelling and the full condition. Confirm that the browser accepts that exact value, not merely the broader property.
- The query passes, but the page still looks wrong. A positive query does not detect bugs or partial implementations. Test the affected behavior in the target environment and adjust the enhancement or provide a targeted fallback.
- The fallback and enhancement spacing are both visible. In the grid example, the baseline uses margins while the enhanced layout uses
gap. Remove or override fallback spacing inside the feature query when both would otherwise apply. - You are adding browser-name branches for a CSS feature. Prefer a capability check. Browser identity is not a reliable substitute for testing the feature itself.
- You added JavaScript for a styling-only decision. Move the condition into
@supports; reserveCSS.supports()for cases where script genuinely needs the boolean result. - The CSS parses but the design is not robust. Parsing support is not a compatibility guarantee. Revisit the baseline, check feature-specific compatibility information, and test the rendered experience.
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