CSS Validator
Validate CSS stylesheets for syntax errors, deprecated properties, and performance issues. Get vendor-prefix suggestions and best-practice warnings. Perfect for front-end developers and designers.
CSS Input
Validation Status
Enter CSS code to see validation results
CSS Best Practices Guide
✓ Best Practices
- • Use CSS custom properties (variables)
- • Keep selectors simple and specific
- • Group related styles together
- • Use shorthand properties when possible
- • Minimize use of !important
- • Use meaningful class names
- • Optimize for performance
- • Include vendor prefixes for new features
- • Use media queries for responsive design
- • Comment your code for clarity
✗ Common Mistakes
- • Overly complex selectors
- • Missing vendor prefixes
- • Using deprecated properties
- • Duplicate or redundant rules
- • Missing units on numeric values
- • Invalid color values
- • Unclosed braces or syntax errors
- • Overusing performance-heavy properties
- • Poor specificity management
- • Inconsistent naming conventions
Sample CSS Code
About CSS Validator
A comprehensive CSS validation tool that analyzes stylesheets for syntax errors (with line and column), deprecated properties, vendor-prefix needs, and performance problems such as duplicate selectors. It is a rule-based checker, not a full validator against the W3C specifications.
Why use a CSS Validator?
Clean, valid CSS is essential for consistent styling across browsers, optimal page load times, and maintainable code. This validator helps identify syntax errors, deprecated properties, and performance issues before they cause styling issues or slow down your website, ensuring professional-quality frontend development.
Who is it for?
Crucial for frontend developers writing custom styles, web designers ensuring cross-browser compatibility, UI/UX professionals optimizing design systems, and teams maintaining large CSS codebases. Perfect for anyone who wants to ensure their stylesheets are efficient, valid, and future-proof.
How to use the tool
Paste your CSS code into the input area (or load one of the built-in examples)
Run the validation to check for syntax errors and compliance issues
Review the reports on deprecated properties and vendor-prefix suggestions
Analyze performance suggestions and optimization recommendations
Apply fixes and re-validate; the Format CSS button rewrites the input with clean formatting
Frequently Asked Questions
How do I validate CSS syntax online?
Paste your CSS into the input area. The validator checks: syntax errors (unmatched braces, missing semicolons, unclosed comments or strings, invalid property names), deprecated properties, vendor-prefix suggestions, a few value checks (colours, missing units), and best-practice warnings. Syntax errors include line/column references. Runs entirely in your browser — your CSS never leaves the device. Useful for: debugging stubborn style issues, learning CSS, pre-deployment code review.
What CSS specifications does this validate against?
It doesn't validate against a formal specification. The checks are hand-written rules: CSS syntax structure, a list of deprecated and non-standard properties, a list of properties that commonly need vendor prefixes, and simple value checks. It does not know every CSS property or every valid value, so it won't flag an unknown property name unless it matches a known typo, and it can't confirm that newer features (Cascade Layers, :has(), Container Queries) are valid. Treat a clean result as 'no problems found by these rules', not as proof of full validity.
Is my CSS sent to a server?
No. The validator runs entirely in your browser, and your CSS is never uploaded.
What kinds of errors will it catch?
Common categories. (1) Syntax errors: unmatched braces, missing semicolons, unclosed comments and strings, missing colons (reported with line and column). (2) Invalid property names: malformed names and a short list of common typos (`colr`, `backgound`, `widht`, `dispaly`). Other unknown property names are not flagged. (3) Value checks: a missing unit on lengths (`width: 300`), suspicious colour values, and empty values. It does not check that a value is valid for its property (`display: flxible` is not flagged). (4) Deprecated properties and values, and vendor-prefix suggestions. (5) Performance warnings: duplicate selectors, complex selectors, heavy properties. There is no browser-target configuration.
What's the difference between validation and linting?
Validation: 'is this syntactically valid CSS?' — yes/no per the CSS specification. Linting: 'does this CSS follow our team's style rules and best practices?' — configurable rules about code quality. Validation catches errors that break parsing; linting catches code-quality issues that work but are suboptimal. Use validation for finding syntax bugs; use a linter for enforcing consistent style across a codebase. Both complement each other.
Why does my CSS work in the browser but fail validation?
Three common causes. (1) Browser tolerance: browsers often render CSS even with minor errors (skipping invalid rules silently). This validator is stricter than browsers. (2) Vendor prefixes: `-webkit-`, `-moz-` properties may be treated differently by different validators. (3) Newer features: cutting-edge CSS (Container Queries, :has(), Cascade Layers) may not be in older validator versions. For features browsers support but validators reject, check the feature's specification status.
How do I fix common CSS errors?
Most common errors and fixes. (1) Missing semicolons: add `;` at end of every declaration. (2) Unmatched braces: count `{` and `}` — they must balance. (3) Typos: `colour` → `color`, `centre` → `center`. (4) Invalid values: check the property's documentation for valid value types. (5) Vendor prefix overuse: modern browsers don't need `-webkit-`, `-moz-` for most properties — check current browser support before adding prefixes. After fixing each error, re-validate to ensure no new issues.
Should I validate CSS in development or only before deployment?
Both. During development: use your editor's built-in CSS validation for immediate feedback. Before deployment: run a linter as a CI/CD step to catch issues before they reach production. For one-off code review, manual ad-hoc checks, or learning CSS, this tool is convenient. For sustained codebase quality, integrate a linter into your build pipeline.
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