Overview
Paste CSS and this tool inserts it into a hidden stylesheet and inspects the browser’s own parsed result (the CSSOM) for signs the parser had to silently drop something - a rule that never registered, or a mismatched brace count that points to an unclosed selector block. Browsers are deliberately forgiving with CSS: an invalid declaration is usually just skipped rather than rejected outright, which is why this tool is upfront about checking rule structure rather than claiming full W3C-spec conformance checking. It’s a distinct job from formatting - a beautifier will happily re-indent CSS with an unclosed brace without ever telling you something is wrong. Runs entirely client-side.
Best for: Catching an unclosed rule that a beautifier would silently reformat around
How to use this tool
- Paste your CSS. Any stylesheet, snippet, or component style block.
- It’s inserted into a real stylesheet. The browser’s own CSS parser processes it via a hidden <style> element.
- The parsed result (CSSOM) is inspected. Rule count and brace balance are checked for signs of a structural break.
- See a clear pass/fail. A specific issue is reported when something looks malformed, not a silent pass.
Why use this tool
Real browser parsing
Uses the same CSSOM every browser builds from your stylesheet, not a reimplemented parser.
Honest about its scope
Flags structural breaks like unclosed rules - it doesn’t claim full W3C conformance checking.
Distinct from formatting
A beautifier reformats around a broken rule silently; this tool is built to flag it instead.
Client-side only
Nothing you paste is uploaded anywhere.
Frequently asked questions
No - browsers are intentionally lenient and simply ignore a declaration they don’t recognize rather than treating the whole stylesheet as broken, so a typo’d property name alone usually won’t be flagged. This tool is aimed at structural breaks - like an unclosed rule or a brace mismatch - that cause the parser to drop content, not a full CSS specification conformance check.
The CSS Formatter re-indents and beautifies CSS but, like most beautifiers, does so on a best-effort basis and won’t reliably flag a broken rule - it’ll often just reformat around it. This tool is purpose-built to answer a different question: is this CSS structurally sound, not just how should it be indented.