Free CSS Validator – Check CSS Syntax & Errors Online

Validate your CSS code instantly with our professional-grade CSS validator. Catch syntax errors, missing semicolons, unknown properties, and common mistakes before they break your website. Real-time feedback with line-by-line error reporting. 100% free, browser-based, and privacy-respecting.

What Is CSS Validation?

CSS validation is the process of checking your Cascading Style Sheets code against standard syntax rules to identify errors, warnings, and potential issues. A CSS validator parses your stylesheet, verifies property names, checks values, and ensures proper structure.

Why CSS Validation Matters for Your Website

Prevent Visual Bugs: Invalid CSS can cause unpredictable rendering across different browsers. A missing semicolon might work in Chrome but break completely in Safari or Firefox. Validation catches these issues before your users do.

Improve Performance: Valid CSS allows browsers to parse stylesheets faster. Syntax errors force browsers to attempt error recovery, slowing down page rendering and potentially affecting Core Web Vitals scores.

Better Maintainability: Clean, validated CSS is easier to debug and modify. When you return to code months later, or when a team member takes over, validated stylesheets are far easier to work with.

SEO Benefits: While CSS validity isn't a direct ranking factor, the resulting performance improvements and consistent rendering do impact user experience metrics that search engines consider.

How to Use This CSS Validator

Step 1: Paste Your CSS

Copy your CSS code from your stylesheet file or code editor and paste it into the input area. The validator accepts any valid CSS, including:

• Standard stylesheets
• CSS modules
• Preprocessor output (compiled Sass/Less)
• CSS-in-JS extracted styles
• CSS custom properties (variables)

Step 2: Review Results Instantly

The validator processes your CSS in real-time as you type. You'll immediately see:

• Green status — Valid CSS with no issues
• Yellow status — Valid but with warnings to review
• Red status — Errors that need fixing

Step 3: Fix Issues Using Line Numbers

Each error and warning includes the exact line number and column position. Click through issues to understand what needs fixing and apply corrections.

Step 4: Download Report

Generate a validation report you can share with team members or keep for documentation purposes.

What This Validator Checks

Syntax Errors

Brace Matching:

/* Error: Unclosed brace */
.header {
  color: blue;
  /* Missing closing brace */

/* The validator detects this immediately */

Semicolon Validation:

/* Error: Missing semicolon */
.element {
  color: red
  background: blue;
}

Empty Values:

/* Error: Empty property value */
.broken {
  margin: ;
}

Property Validation

The validator checks against 200+ valid CSS properties including:

• Box model (margin, padding, border, width, height)
• Flexbox (display, flex-direction, justify-content, align-items)
• Grid (grid-template-columns, grid-area, gap)
• Typography (font-family, font-size, line-height, text-align)
• Visual (color, background, box-shadow, opacity)
• Animation (transition, animation, transform)
• Modern CSS (aspect-ratio, backdrop-filter, scroll-snap)

Unknown Property Warning:

/* Warning: Unknown property */
.element {
  colr: red; /* Typo - should be "color" */
}

Best Practice Warnings

Double Semicolons:

.element {
  margin: 10px;; /* Unnecessary double semicolon */
}

Double Braces:

.element {{  /* Syntax error */
  color: blue;
}}

Unclosed Comments:

/* This comment never closes
.element {
  color: red;
}

Common CSS Errors and How to Fix Them

Missing Semicolons

Problem: CSS declarations must end with semicolons. Missing one can cause the next property to be ignored.

Solution: Always end each property-value pair with a semicolon:

.correct {
  color: blue;
  margin: 10px;
  padding: 5px;
}

Typos in Property Names

Problem: Misspelled properties are silently ignored by browsers, making bugs hard to find.

Solution: Use the validator to catch typos:

/* Wrong */
.element {
  backgrund: blue;  /* Typo */
}

/* Right */
.element {
  background: blue;
}

Invalid Values

Problem: Using wrong value types for properties.

Solution: Match values to property expectations:

/* Wrong */
.element {
  width: blue;  /* Color value for size property */
}

/* Right */
.element {
  width: 100px;
}

Unclosed Braces

Problem: Missing closing braces break all subsequent styles.

Solution: Use proper indentation and always close what you open:

.parent {
  color: blue;
}  /* Don't forget this */

.child {
  color: red;
}

CSS Validation vs CSS Linting

Both are valuable. Validation ensures your CSS works; linting ensures it follows team standards.

Supported CSS Features

Standard CSS

All CSS1, CSS2, and CSS3 properties and selectors.

CSS Custom Properties

:root {
  --primary-color: #3b82f6;
  --spacing: 1rem;
}

.element {
  color: var(--primary-color);
  margin: var(--spacing);
}

Modern CSS

• Container queries
• Cascade layers (@layer)
• :has() and :is() selectors
• Logical properties
• Aspect ratio
• Scroll snap

Vendor Prefixes

.element {
  -webkit-transform: rotate(45deg);
  -moz-transform: rotate(45deg);
  transform: rotate(45deg);
}

Frequently Asked Questions

Does this validator check CSS3?

Yes. Our validator supports all modern CSS3 properties including Flexbox, Grid, Custom Properties, animations, and more.

Will my CSS be sent to a server?

No. All validation happens locally in your browser. Your code never leaves your device.

Can I validate preprocessor syntax (Sass/Less)?

This validator works with compiled CSS output. For Sass or Less syntax, compile first, then validate the CSS output.

What's the difference between errors and warnings?

Errors indicate invalid syntax that will cause issues. Warnings highlight potential problems or unknown properties that might be intentional.

Does it validate vendor prefixes?

Yes. Common vendor prefixes (-webkit-, -moz-, -ms-, -o-) are recognized as valid.

Can I validate CSS from a URL?

Currently, you need to paste the CSS directly. Copy the stylesheet content and paste it into the validator.

Related Tools

• CSS Formatter — Beautify CSS code
• CSS Minifier — Compress CSS
• Autoprefixer — Add vendor prefixes
• HTML Validator — Validate HTML
• JSON Validator — Validate JSON

Paste your CSS above to validate instantly. Get line-by-line feedback and download validation reports for your team.