CSS Validator
Check CSS for syntax and structural issues directly in your browser.
How to validate CSS
- Paste CSS in the input panel.
- Click Validate CSS.
- Review the summary and issue list.
What this validator checks
- Unclosed comments, strings, url( functions.
- Balanced braces, parentheses, brackets.
- Declarations missing a top-level colon.
Why parsing is not the same as validation
A successful parse tells you the browser produced output, not that the input was valid.
What this validator does not check
- Whether property values are valid for their properties.
- Whether a CSS feature is supported by a specific browser.
- CSS, JavaScript, SVG, MathML content.
When CSS validation is useful
Running a stylesheet through validation is a fast way to catch a mistyped brace or a malformed declaration before it causes confusing layout problems, and it's a good habit to build into reviewing any stylesheet before it ships, especially one that's been hand-edited or pieced together from multiple sources.
Common CSS errors
- Missing braces - a rule that opens with a brace but never closes it, which can silently swallow every rule that follows.
- Invalid declarations - a property written without its colon, or a stray semicolon in the wrong place.
- Malformed values - an unclosed string, an unterminated url() function, or unbalanced parentheses inside a value.
- Selector mistakes - an unclosed bracket in an attribute selector, or a selector list left dangling without its rule body.
Syntax validation vs. browser support and visual correctness
This validator confirms the CSS is structurally sound - the kind of check that catches typos and broken syntax - but it doesn't verify that a property, value, or selector actually does what you intend in a browser. A stylesheet can pass validation and still render incorrectly because of a browser compatibility gap, an unsupported value, or a specificity conflict elsewhere in the cascade. Some CSS features, like newer color functions, container queries, or nesting syntax, depend on browser and version support that validation alone can't confirm - checking a compatibility reference for anything unusual is worth doing alongside validation.
Frequently asked questions
- Does this CSS validator upload my code?
- No. Parsing and validation run entirely in your browser.
- What does this validator check?
- Unclosed comments, strings, braces, parens, brackets, and declarations missing a property colon.
- Does it support CSS nesting?
- Yes. Nested rules are parsed and checked using the same structural rules as top-level rules, so unclosed braces or malformed selectors inside a nested block are still caught.
- Can valid CSS still fail to display correctly in a browser?
- Yes. Syntax validation confirms the stylesheet is structured correctly - matching braces, well-formed declarations - but says nothing about whether a given browser supports a particular property, function, or selector, or renders it the way you'd expect. A rule can be perfectly valid CSS and still be ignored by a browser that doesn't support it.
- What are common mistakes this validator catches?
- A missing closing brace at the end of a rule, a declaration written without its colon, an unclosed string or url() value, and mismatched or unbalanced parentheses and brackets are among the most frequent syntax mistakes flagged.