Use a reliable baseline first, then put CSS enhancements behind an @supports check for the exact capability they need. This is the core of how to use CSS feature detection for cross-browser compatibility: ask whether a browser accepts a CSS feature, rather than guessing from its name or version. A passing check is not proof that the feature works perfectly, so pair it with compatibility data and testing in the browsers your users rely on.
Build a baseline, then enhance it with @supports
A CSS feature query conditionally applies styles when the browser considers a declaration valid. Keep content and useful fallback styles outside the query; add the enhanced layout inside it.
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/* Baseline layout remains usable if grid is unavailable. */
.cards {
display: block;
}
.cards > * + * {
margin-block-start: 1rem;
}
@supports (display: grid) {
.cards {
display: grid;
grid-template-columns: repeat(3, minmax(0, 1fr));
gap: 1rem;
}
.cards > * + * {
margin-block-start: 0;
}
}In a browser that accepts display: grid, the grid rules apply and the earlier spacing rule is reset. If it does not, the block layout and spacing remain. This progressive-enhancement pattern is usually clearer and safer than making the newer feature the only way the page works.
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Check the feature your enhancement actually depends on
Test the declaration or condition that matters, not a loosely related feature. For example, testing display: grid says the browser accepts that declaration; it does not establish support for every Grid value or behavior your design may use. If the enhancement depends on a particular value, include that value in the condition:
@supports (display: grid) and (grid-template-columns: subgrid) {
.nested {
display: grid;
grid-template-columns: subgrid;
}
}Conditions can be combined with and, or, and not. Selector support can also be checked, for example @supports selector(:has(a)). The supports() function can condition an @import; see MDN’s @import reference for its syntax.
Use @supports not only when the fallback needs a separate rule
You can write a targeted alternative with @supports not (display: grid). But if the regular cascade already gives unsupported browsers a good baseline, a second branch adds duplication without improving the result. Add a negative query when it genuinely needs different styling or behavior.
Remember that unknown declarations are ignored
Browsers ignore declarations they do not recognize. You do not need a feature query around every newer CSS property: it is most useful when several enhanced declarations belong together, when the fallback must change, or when an explicit conditional boundary makes the stylesheet easier to understand.
Use CSS.supports() when JavaScript needs the answer
If JavaScript must choose whether to run code or activate a stylesheet based on CSS capability, use the CSS.supports() API. It returns a boolean and accepts either a property/value pair or a supports-condition string.
if (CSS.supports("grid-template-columns", "subgrid")) {
// Load or activate behavior that depends on subgrid.
}For a CSS-only decision, prefer @supports so the styling stays in CSS and does not require JavaScript. See MDN’s CSS.supports() reference.
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Know what a positive feature query does—and does not—prove
A passing query means the browser considers the tested syntax valid. It does not guarantee the implementation is complete, correct, or free of browser-specific bugs, and feature queries cannot detect partial implementations. Do not treat @supports as a guarantee of cross-browser compatibility.
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For a behavior-sensitive feature, combine three checks that answer different questions:
- Feature query: Does the browser accept the tested syntax or condition?
- Compatibility information: Which browser versions are reported to support this exact feature? Check the relevant MDN CSS compatibility information.
- Browser testing: Does the page render and behave as intended in the environments you support?
MDN’s cross-browser testing introduction explains the testing task, and Playwright’s browser documentation describes testing across browser engines. Availability varies by feature and browser release, so check the exact capability and the environments relevant to your site.
Choose between CSS, JavaScript, and browser testing
| Need | Use | Why |
|---|---|---|
| Apply CSS only when a declaration or selector is supported | @supports |
Keeps a CSS decision in the stylesheet and lets the baseline cascade work without the enhancement. |
| Let JavaScript branch on CSS capability | CSS.supports() |
Returns a boolean for a property/value pair or supports condition. |
| Find out whether an implementation is buggy, incomplete, or visually wrong | Compatibility information plus tests in target browsers | A syntax-acceptance check cannot establish correct behavior. |
Avoid user-agent sniffing as a substitute for capability checks. Browser identity is an indirect guess about support; a feature query asks about the capability itself. For a known implementation bug, use compatibility information and test the affected browser versions rather than assuming a passing query resolves it.
Verify the rendered result in the browsers that matter
- List the browsers and devices your site needs to support.
- Check compatibility information for the exact CSS feature and values used, rather than for a related property.
- Test the fallback and enhanced result in the target environments, including the visual or interactive behavior that matters.
- If a query passes but the feature behaves incorrectly, treat that as an implementation limitation: revise the enhancement, add a targeted fallback, or avoid relying on the feature in that environment.
Troubleshoot feature detection
- The enhancement never applies: Check the condition’s syntax and confirm it tests the exact property/value or selector used. Verify that the target browser accepts that condition.
- The query passes but the layout is wrong: Acceptance is not a behavior test. Check compatibility notes and reproduce the result in the affected browser; add a fallback or adjust the enhancement if needed.
- The fallback is broken even when the feature is unavailable: Move essential layout and spacing rules outside the query, then limit the query to enhancements.
- JavaScript reports an unexpected result: Ensure the property and value are passed as separate strings, or supply a valid supports-condition string. Keep CSS-only styling decisions in
@supports. - The page works in one browser but not another: Test each target engine and device. A browser-name assumption or a passing query alone cannot confirm the rendered experience.
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Frequently Asked Questions
Does a passing @supports check guarantee the CSS will work?
No. It confirms the browser considers the tested condition valid; it does not detect every incomplete implementation or browser bug. Test behavior in the target environments.
Should I use JavaScript or CSS for feature detection?
Use @supports for styling decisions and CSS.supports() when JavaScript itself must branch on CSS capability.
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