CSS Validator

CSS Validator: Check Your CSS Online

This CSS validator checks your stylesheet directly in the browser and reports common errors with line numbers and severity. It detects unbalanced braces, missing colons, empty values, empty rules and unknown or unusual CSS properties against a large whitelist. Find and fix errors quickly before your code goes into production.

The tool is a practical syntax and sanity checker, not a full W3C validator. It checks the structure of your CSS file for common problems and helps you understand the basics of CSS syntax. For complete W3C conformance you can use the official W3C CSS Validator online.

What does it check?

• Brace balance: Missing or extra curly braces as well as braces inside values are detected.
• Declarations: The format „property: value;" is checked, including the colon and filled-in value.
• Properties: Matching against a whitelist of common CSS properties, including CSS variables with the -- prefix.
• Empty rules: Selector blocks without declarations are flagged as a hint.

What it can and cannot do

The validator reliably detects syntax errors and shows the affected line number. It can identify missing closing braces, invalid property names and missing values. However, it cannot detect semantic errors, evaluate browser-specific behavior, or validate the correctness of values. It is a tool for narrowing down problems, not a complete solution.

Why local processing in the browser

The check runs entirely in your browser. Your CSS code is not transmitted to our server. This means full GDPR compliance: no accounts, no sign-up, no data transmission. Enter your code, fix errors, and close the browser without leaving a trace of your work. Everything stays on your device.

Creative usage ideas

• Pre-deploy check: Verify your CSS code for syntax errors before pushing to production.
• Learning CSS syntax: Understand what errors the validator reports and learn how correct CSS is structured.
• Cleaning up old projects: Analyze legacy stylesheets and identify outdated or unknown properties.
• Code review: Use the tool as a first filter before a deeper review to find obvious errors.
• Workflow integration: Integrate the checker into your development workflow for continuous syntax checking.
• Teaching aid: Show students and learners how CSS errors occur and how to find them systematically.

A small contribution to climate action

Every check on your device avoids a server request over the Internet. A CSS check consumes only a fraction of the energy needed for a data transfer. The International Energy Agency estimates that data centers consumed about 415 terawatt-hours in 2024, roughly 1.5 percent of global electricity demand. Local tools reduce network traffic and energy consumption.

Frequently asked questions