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ValidateHTML

Free CSS Validator

Paste your CSS code to check for syntax errors, invalid properties, and incorrect values. Get a quality score with detailed reports.

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Ctrl+Enter to validate
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Syntax Checking

Catch syntax errors, missing semicolons, unclosed brackets, and malformed declarations instantly.

Property Validation

Verify that CSS properties and values are valid according to the latest CSS specification.

Quality Score

Get a 0-100 score and letter grade for your CSS quality. Track improvements as you fix issues.

Common CSS Errors

Missing Semicolons

Forgetting a semicolon at the end of a declaration causes the next property to be ignored or misinterpreted.

Invalid Property Names

Typos in property names like 'colr' instead of 'color' are silently ignored by browsers, making bugs hard to find.

Invalid Values

Using incorrect values like 'display: flexbox' instead of 'display: flex' won't produce errors in the browser, just broken layouts.

Unclosed Brackets

Missing closing braces cause all subsequent rules to be nested inside the wrong selector.

Missing Units

Numeric values (except 0) require units. 'width: 100' won't work, use 'width: 100px' or 'width: 100%'.

Invalid Selectors

Malformed selectors like '.class.' or '#id#id' are silently discarded by the browser.

Why Validate Your CSS?

Unlike HTML, CSS fails silently. Invalid properties and values are simply ignored by browsers without any error messages. This makes CSS bugs particularly hard to track down. A typo in a property name can cause hours of debugging.

Validating your CSS catches these silent failures before they reach production. It helps you identify typos, deprecated properties, browser compatibility issues, and syntax errors that could break your layout.

Clean, valid CSS also improves page performance. Invalid declarations force browsers to do extra parsing work, and redundant or conflicting rules increase file size without benefit.

Frequently Asked Questions

What CSS errors does this validator check for?
Our CSS validator checks for syntax errors, invalid property names, incorrect property values, malformed selectors, missing semicolons, unclosed brackets, invalid at-rules, and more. We validate against the latest CSS specification and flag both hard errors and best-practice warnings.
Is this CSS validator free?
Yes, our CSS validator is completely free to use. No registration required, no limits on usage. Paste your CSS and validate instantly.
What is the difference between this and the W3C CSS validator?
Our validator provides a modern, faster experience with a quality score and detailed error explanations. The W3C validator is the official standard but has a dated interface. Both check against CSS specifications, but our tool gives you actionable results with a cleaner UX.
Does CSS validation affect SEO?
Not directly. Google does not penalize invalid CSS, but broken CSS can cause rendering issues that hurt Core Web Vitals (LCP, CLS) and mobile usability, which ARE ranking factors. Clean CSS also loads faster, which indirectly improves SEO.
What is a good CSS quality score?
A score of 90-100 (Grade A) means your CSS follows best practices with no significant issues. 75-89 (Grade B) is solid with a few minor issues. Below 50 indicates broken syntax or many specificity problems. Most production stylesheets score 70-90.
Why does my browser render invalid CSS without errors?
Browsers are intentionally lenient with CSS. When they encounter an invalid property, value, or selector, they simply skip it instead of breaking the whole stylesheet. This design choice keeps websites working when browsers add new features, but it means typos and bugs can go unnoticed without a dedicated validator.
Can I validate SCSS or Less files?
This tool validates compiled CSS output. Paste your compiled .css file, not the .scss or .less source. SCSS/Less preprocessors have their own syntax that is not part of the CSS specification, so they need to be compiled first.