Does this send my stylesheet anywhere?

No. Validation runs entirely in your browser.

Does this check for unsupported browser features?

No. It's a syntax check, not a browser-compatibility check like caniuse.

Does it check SCSS or LESS syntax?

No. It uses Prettier's plain CSS parser. SCSS and LESS need Prettier's separate scss and less parsers, so preprocessor-specific syntax like nesting or SCSS variables can report a false error here.

What's the difference between this and the CSS Formatter?

They share the exact same Prettier CSS parser. This tool only reports pass/fail plus the error location; the Formatter also reformats the input once it's confirmed valid.

CSS Validator - Check CSS Syntax Online

Overview

Introduction

A CSS syntax error, like an unclosed brace, can silently break the rest of a stylesheet in ways that are hard to spot by eye.

This tool checks CSS syntax entirely in your browser and reports the exact location of the first problem, if any.

What Is CSS Validator?

A syntax validator that parses your stylesheet with Prettier's PostCSS-based CSS parser and reports success or the first parse error.

It checks structural validity, not whether property values or selectors follow best practices.

How CSS Validator Works

The parser attempts to build a syntax tree from your stylesheet.

If parsing succeeds, the CSS is valid; if it fails, the exact line and column of the problem is shown.

When To Use CSS Validator

Use it when a stylesheet isn't applying as expected and you want to rule out a syntax error before debugging specificity or cascade issues.

It's also useful for a quick pass/fail check on LLM-generated or copy-pasted CSS before adding it to a project.

Often used alongside CSS Formatter and CSS Minifier.

Features

Advantages

Runs entirely client-side.
Reports the exact line and column of the first syntax problem.
Uses the same Prettier parser relied on by the CSS Formatter, rather than a bespoke or partial parser.

Limitations

Only checks syntax, not unknown properties, unsupported values, or browser compatibility.

Examples

Unclosed rule

Input

.btn {
  color: red;

Output

Unclosed block

The parser reports that a rule was opened but never closed with a matching brace.

Best Practices & Notes

Best Practices

Use this before the CSS Formatter when you specifically want a fast pass/fail syntax check without reformatting anything.
If validation fails on SCSS or LESS input, remember this checks plain CSS syntax; preprocessor-specific syntax isn't supported here.

Developer Notes

Reuses the same Prettier `css` parser as the CSS Formatter; the input is discarded on success and only the parse result matters.

CSS Validator Use Cases

Ruling out a syntax error when styles aren't applying as expected
Checking a generated or LLM-produced stylesheet before using it

Common Mistakes

Assuming a 'valid' result means every property and value is supported by browsers; it only confirms the syntax parses.
Pasting SCSS or LESS source and expecting it to validate; only standard CSS syntax is checked.

Tips

If validation fails unexpectedly, check for a missing semicolon on the line just above the reported error.
For unknown properties or typo'd values, which still parse as valid CSS, a linter like Stylelint catches issues this syntax check won't.

CSS Validator