CSS Validator
LiveFree online CSS validator and checker — validate CSS syntax, catch errors, and lint your stylesheets instantly with detailed error reporting.
Understanding Cascading Style Sheets
CSS validation catches syntax errors, invalid property names, unsupported values, and specification violations that cause styles to silently fail. Unlike JavaScript which throws runtime errors, CSS simply ignores invalid rules — a typo in a property name (colr instead of color) or an invalid value (display: flexbox instead of display: flex) produces no error message in the browser. The property is silently dropped, and developers wonder why their styles are not applying. CSS validators catch these silent failures before they reach production.
A free online CSS validator and syntax checker. Paste your CSS code and instantly check for syntax errors, unclosed brackets, invalid properties, malformed selectors, and common mistakes. Get detailed error feedback with line numbers and suggestions. Works as a CSS checker to validate stylesheets and a code validator for your styles. Whether you need to validate CSS syntax before deployment or lint your stylesheets — this tool catches issues fast. Supports modern CSS including Grid, Flexbox, custom properties, :has(), and :is(). No signup, 100% client-side.
The Devkitr CSS Validator checks your stylesheets against CSS specifications to identify invalid properties, unsupported values, syntax errors, and deprecated features. Paste CSS to get a list of all issues with line numbers, severity levels, and fix suggestions — catching the silent failures that browser DevTools do not flag.
In a typical development workflow, CSS Validator becomes valuable whenever you need to free online css validator and checker. Whether you are working on a personal side project, maintaining production applications for a company, or collaborating with a distributed team across time zones, having a reliable browser-based validation tool eliminates the need to install desktop software, write one-off scripts, or send data to third-party services that may log or retain your information. Since CSS Validator processes everything locally on your device, your data stays private and your workflow stays uninterrupted — open a browser tab, paste your input, get your result.
Key Features
Property Validation
Verifies that every CSS property name exists in the specification and that its value is valid for that property type.
Value Range Checking
Catches invalid values like negative widths, opacity above 1.0, and z-index values that exceed browser limits.
Deprecation Warnings
Flags deprecated CSS features with modern alternatives — like -moz-appearance vs appearance, or -webkit-transition vs transition.
Selector Validation
Checks selector syntax for invalid pseudo-classes, unclosed attribute selectors, and other selector-level syntax errors.
How to Use CSS Validator
Paste Your CSS
Enter the stylesheet or CSS snippet you want to validate against specification rules.
Review Errors
Each error lists the invalid property or value, its line number, and what the specification expects instead.
Check Warnings
Warnings for deprecated features and browser-prefixed properties suggest modern standards-compliant alternatives.
Fix and Re-validate
Correct the identified issues and re-validate to confirm your CSS is clean and specification-compliant.
Use Cases
Debugging Styles That Are Not Applying
Validate CSS to find the typo when a style silently fails — CSS does not throw errors, so invalid properties are simply ignored.
Cleaning Up Legacy Stylesheets
Validate old CSS files to identify deprecated features, vendor prefixes that are no longer needed, and invalid values from outdated practices.
Ensuring Cross-Browser Compatibility
Validate CSS against the specification to ensure styles work across all browsers, not just the one you test in.
Reviewing Auto-Generated CSS
Validate CSS produced by tools, preprocessors, or AI generators to catch specification violations before deployment.
Pro Tips
Validate CSS after preprocessor compilation (SCSS→CSS) to catch issues introduced during compilation that the preprocessor did not flag.
Use the validator to identify browser-prefixed properties that can be removed — autoprefixer adds prefixes, but old manually-added prefixes may persist.
Run validation on critical CSS (above-the-fold styles) separately to ensure the most important styles are error-free.
Combine CSS validation with browser DevTools "Rendering" panel to catch paint issues that valid CSS can still cause.
Common Pitfalls
Assuming CSS is correct because the page renders without visible issues
Fix: CSS silently ignores invalid rules. The page may look fine because of fallback or inherited styles, while your intended styles are being dropped.
Using CSS custom property names that conflict with existing properties
Fix: Custom properties (--color, --size) should use descriptive names that cannot be confused with standard CSS properties or browser internals.
Not validating CSS inside HTML style attributes
Fix: Inline styles in style="" attributes can also contain errors. Extract inline CSS and validate it separately, especially in email templates.
Frequently Asked Questions
QWhat does this CSS validator check?
It catches unclosed brackets, missing semicolons, invalid property names, malformed selectors, and other CSS syntax violations. Get detailed error messages with line numbers to fix issues quickly.
QDoes it support modern CSS?
Yes. CSS Grid, Flexbox, custom properties (variables), modern selectors like :has() and :is(), container queries, and other current CSS features are fully recognized by the validator.
QHow is this different from W3C validators?
This CSS validator runs 100% in your browser with no signup required. Your code is never sent to any server, making it faster and more private than online validators that require uploading your code.
QDoes it check browser compatibility?
No. It validates CSS syntax correctness only. Use caniuse.com for checking CSS property browser compatibility.
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