To create HTML and CSS that work across browsers, start with semantic, valid markup and a usable baseline, check support for each important feature against the browsers your audience uses, add newer capabilities as enhancements with fallbacks, and test the result in those browsers. No single compatibility label or feature check can guarantee correct behavior everywhere.
Define which browsers you need to support
“Browser-compatible” needs a target. List the browsers and versions your audience actually uses, along with relevant phones, tablets, and embedded web views. Then decide which parts of the experience are essential and must work in that target set, and which can be enhancements.
MDN Baseline is a useful starting point for understanding support across its covered browsers, but it does not cover every older release, embedded web view, or assistive technology. Treat it as a summary, not as your complete support policy.
Build a semantic, usable baseline
Write HTML for meaning and structure, then make sure the page’s essential content and interactions remain usable without relying on newer styling. Progressive enhancement follows this baseline-first approach: broadly available functionality comes first; richer behavior is added where supported.
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Keep the document structure valid. An HTML document has one root <html> element, with the rest of its elements beneath it. Browsers may repair malformed markup while rendering, so a page that looks acceptable is not proof that its HTML is valid. Run a validator as part of development.
Keep core styles outside conditional feature queries. That gives browsers that do not support an enhancement a functional starting point rather than an empty or broken layout.
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Check support for each important CSS feature
Compatibility is feature-specific: a browser may support one modern layout feature but not another. For each property or behavior that materially affects layout or interaction, check current compatibility data for the exact feature and consider the versions, devices, and embedded contexts in your support target. MDN’s cross-browser testing guide explains how to investigate unsupported features and layout differences.
When a feature is unsupported, choose a simpler baseline that preserves access to the content and essential functionality. Add the newer layout as a progressive enhancement, for example:
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.cards {
display: block;
}
@supports (display: grid) {
.cards {
display: grid;
gap: 1rem;
}
}The first rule provides a basic layout. The feature query adds grid styling when the browser understands the tested declaration. See MDN’s feature query reference for the behavior and limits of @supports.
Use capability detection, not browser-name branching
When your page depends on a capability, test for that capability instead of assuming support from a browser’s name or user-agent string. CSS can use @supports; JavaScript can check for the relevant property or method. This focuses the decision on what the page needs, rather than on a browser label that may not reliably describe what is available. MDN covers feature detection and why it differs from browser detection using the user agent.
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A positive feature query only tells you that the browser recognizes the tested declaration. It does not establish that the feature is fully implemented, bug-free, or behaves as your page requires. Keep a fallback and test the actual behavior.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Validate and test in the browsers that matter
- Validate the HTML. Fix structural and syntax errors instead of relying on a browser’s automatic repair.
- Check compatibility data. Look up each important CSS feature and the precise property/value combination against your support target.
- Load the page in multiple target browsers. Test real layouts and interactions, including the devices and embedded contexts relevant to your audience.
- Compare any discrepancy with the applicable specification and compatibility information. This helps distinguish missing support from a limitation or possible browser bug.
- Reduce unexpected behavior to a small reproducible example. Retest that case across the relevant browsers before changing the implementation.
MDN’s testing guide discusses validation, compatibility data, and common layout issues. For unexpected differences, its browser bug guide recommends checking multiple browsers, compatibility tables, and specifications.
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Or skip the browser setup
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation for request options. Screenshot capture can help inspect appearance, but it does not replace validating markup, checking feature support, or testing interactions in actual target browsers.
- Cookie and consent banners, newsletter popups, and chat widgets are removed before capture; each step can be turned off.
- Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed. Responses identify the page verdict and billing status in headers.
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Frequently Asked Questions
Does a passing @supports test guarantee the CSS will work correctly?
No. It shows that the browser understands the tested declaration, not that its implementation is complete or bug-free. Test the behavior in your target browsers.
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Should I use user-agent detection to decide whether to apply a CSS feature?
Prefer capability detection: use @supports for CSS declarations or check the relevant property or method in JavaScript. Browser names do not directly establish the capability your page needs.
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