What does a CSS validator do?

A CSS validator parses your stylesheet and reports syntax errors — unmatched braces, missing semicolons, malformed declarations, invalid at-rules. It catches mistakes a browser silently ignores, helping you ship clean, predictable CSS.

What does CSS validation NOT check?

Validators check syntax, not behaviour. They will not flag invalid property values for the wrong context, browser-prefix coverage, accessibility issues (colour contrast, focus styles), or whether selectors actually match elements in your HTML. Use Lighthouse or axe for those.

What are the most common CSS syntax errors?

Missing closing braces, semicolons forgotten between declarations, mismatched parentheses in calc() or rgba(), unterminated comments, and invalid property names (typos like "colour" instead of "color"). Modern browsers silently skip these, hiding the bug.

Does this tool work for SCSS or Less?

No — this validator targets plain CSS only. SCSS and Less have additional syntax (variables, nesting, mixins) that plain CSS validators flag as errors. Use a Sass/Less compiler to validate preprocessor files.

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CSS Validator

Validate

Validate CSS syntax in your browser. Catch unclosed braces, bad properties and missing semicolons.

Input

Loading editor…

✔

Paste CSS then click Validate.

Parsed entirely in your browser — no server needed.

What Is a CSS Validator?

A CSS validator parses your stylesheet and reports syntax errors with exact line and column locations. It catches missing semicolons, unclosed braces, invalid property syntax, malformed selectors, and bad at-rule syntax — the kinds of mistakes that cause a browser to silently skip part of your stylesheet. Catching these early saves debugging time later when a page mysteriously renders wrong.

CSS Validator Online — What This Tool Does

This free CSS validator parses your CSS entirely in your browser using Prettier's CSS parser. If valid, you get a structural summary — rule count, declarations, at-rules. If invalid, you get the exact parse error with line and column number. No upload, no signup, no server-side processing.

How to Validate CSS Online

Paste CSS into the Input pane, or click Upload to load a .css file.
Click Validate — results appear in the right panel.
If valid, see the stats summary: rule count, declarations, and at-rules.
If invalid, the exact parse error with line/column is shown.
Fix the source CSS in the input pane and re-validate.

What Does the Validator Check?

Unclosed braces — every { needs a matching }.
Missing semicolons — declarations must end with ; (technically the last one before } is optional but recommended).
Invalid property syntax — values that don't match the property's expected format.
Malformed selectors — bad combinators, invalid pseudo-class syntax.
Bad at-rule syntax — wrong @media, @keyframes, @font-face, @import usage.
Unmatched parentheses — common in calc(), var(), and linear-gradient().

What This Validator Doesn't Check

This tool validates syntactic correctness — whether the CSS parses. It does not check semantic issues like unknown property names (colorz: red parses fine but doesn't apply), browser compatibility, or accessibility. For those, use caniuse.com, stylelint, or a CSS coverage tool like Chrome DevTools' Coverage panel.

Common CSS Errors and How to Fix Them

Unclosed brace — count { and }. The validator reports the line where the parser got confused, but the real error is usually earlier.
Missing semicolon — most common in multi-line declarations. Add ; at the end of every property line.
Unquoted url — modern parsers accept url(image.png) without quotes, but with special characters use url("image.png").
Invalid colour — typos like #ZZZ or rgb(256, 0, 0) (max is 255) fail validation.
Bad calc() syntax — operators in calc() require spaces: calc(100% - 20px) not calc(100%-20px).

Tips & Tricks

Validate before committing — catches sloppy edits before they become production bugs.
For larger projects, use stylelint — it catches semantic issues this validator misses (unknown properties, accessibility, project-specific rules).
Pair with the CSS Formatter — format first to make errors easier to spot visually.
Browser DevTools as a complementary check — Chrome highlights invalid CSS in the Styles panel with a strikethrough.

Related Tools

CSS Formatter — beautify and format CSS stylesheets.
CSS Minifier — compress CSS for production.
SCSS Formatter — format SCSS files with Prettier.
HTML Validator — validate HTML markup.
JSON Validator — validate JSON syntax.

Is My Code Sent to a Server?

No. CSS validation runs entirely in your browser using Prettier's CSS parser. Your CSS — including any proprietary design tokens — never leaves your machine. There is no upload, log, or analytics on the CSS you validate.