To create browser-compatible HTML and CSS, start with valid semantic markup and a usable baseline, then add newer features as enhancements. Check support for each feature in the browsers and devices your audience actually uses, and test the finished page in those environments. A CSS feature query can help choose a fallback, but it cannot prove that a browser implements a feature correctly.
1. Decide which browsers and devices you need to support
“Works in all browsers” is not a practical target: browser versions, embedded web views, devices, and assistive technologies differ. Write down the browsers, versions, screen sizes, and contexts that matter for your audience before choosing a layout or interaction.
MDN Baseline offers a useful starting summary of compatibility across popular browsers, but it does not cover every older release, web view, or assistive technology. Treat it as a signal for planning, not as a complete support policy.
2. Build a semantic HTML baseline
Use elements for their meaning, keep essential content and actions available without advanced styling, and make sure your document has a single root <html> element with the rest of the document nested beneath it. Meaningful structure gives browsers and assistive technologies information that visual styling alone cannot supply.
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Validate your markup rather than relying only on how it looks: browsers may repair malformed HTML while rendering, so a page can appear normal and still contain errors. For guidance on checking HTML and CSS, see MDN’s testing guide and the MDN reference for the <html> element.
3. Check support for each CSS feature
Compatibility is feature-specific. For a layout or interaction that matters, look up the exact CSS feature and property/value combination against your target browsers. A general statement that a browser is modern does not answer whether every feature you use is supported, especially in older versions or on less common devices. MDN’s testing guide explains how compatibility data can help identify unsupported features and layout issues.
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4. Add enhancements without making them a requirement
Keep the baseline outside a feature query, then conditionally apply the enhancement. This example uses ordinary block flow as a fallback and CSS Grid where the browser recognizes it:
.cards {
display: block;
}
@supports (display: grid) {
.cards {
display: grid;
gap: 1rem;
}
}The baseline-first approach is progressive enhancement: core content and functionality remain available broadly, while richer behavior is layered on where supported. See MDN’s progressive enhancement glossary.
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CSS @supports tests whether a user agent understands a declaration. A positive result does not detect implementation bugs, spec violations, or partial implementations. Keep the fallback useful and test the enhanced behavior in the browsers you support.
5. Detect capabilities, not browser names
When code needs a capability, test for that capability and provide an alternative if it is unavailable. CSS can use @supports; JavaScript can check for the relevant property or method. Browser-name or user-agent branching is brittle for this purpose because it guesses what a browser can do rather than checking the needed behavior. See MDN’s browser detection guidance and feature detection guide.
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6. Test in the browsers that matter
- Validate the HTML and CSS. Fix structural and syntax problems before diagnosing browser-specific rendering.
- Load the page in more than one target browser. Include relevant device sizes and embedded contexts, not just the browser you use to develop.
- Check the exact feature’s compatibility data. Compare what you observe with documented support for the feature and target version.
- Reduce discrepancies to a small reproducible case. Determine whether the cause is unsupported behavior, an implementation limitation, or a browser bug; consult the applicable specification and compatibility tables. MDN’s browser compatibility bug guide recommends this kind of cross-browser checking.
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Frequently Asked Questions
Does `@supports` confirm that a CSS feature works correctly?
No. It checks whether the browser understands the declaration, not whether its implementation is bug-free or complete. Test the behavior in your target browsers.
Should I use user-agent detection to choose CSS?
Prefer checking the capability the page needs. Browser-name and user-agent rules do not reliably establish support for a particular feature.
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