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What Is CSS Validation?
CSS validation checks whether your stylesheets conform to the CSS specification. Valid CSS ensures consistent rendering across browsers, prevents unexpected layout shifts, and catches syntax errors that can silently break your designs.
CSS is forgiving by design — browsers skip rules they do not understand rather than throwing errors. This means invalid properties, typos in values, and incorrect selectors can go unnoticed for months. A CSS validator catches these problems immediately.
How the CSS Validator Works
CheckTown's CSS validator parses your stylesheet and reports errors with line numbers and property-level details.
- Syntax checking — detects invalid selectors, malformed rules, and missing semicolons or brackets
- Property validation — verifies that property names exist in the CSS specification and values are valid for each property
- Vendor prefix warnings — identifies non-standard vendor prefixes and suggests standard alternatives when available
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Validate Your CSS →Common CSS Errors and How To Fix Them
CSS errors are often invisible because browsers silently ignore invalid rules. These are the most common mistakes that validators catch.
- Invalid property values — using an incorrect value type for a property, such as applying a color name where only numeric values are accepted
- Missing units — forgetting to add px, em, rem, or other units to numeric values where they are required
- Unclosed brackets — a missing closing brace can cause every subsequent rule to be ignored by the browser
When To Use CSS Validation
CSS validation helps maintain clean, maintainable stylesheets throughout the development lifecycle.
- Before deployment — validate CSS to catch errors that are invisible during development but affect users in production
- Debugging layout issues — when a layout behaves unexpectedly, validation often reveals the root cause faster than manual inspection
- Code reviews — include CSS validation as a quality gate in pull request workflows
Frequently Asked Questions
Does the validator support modern CSS features like CSS Grid and custom properties?
Yes. The validator supports current CSS specifications including Grid, Flexbox, custom properties (CSS variables), and media queries. It validates against the latest W3C recommendations.
Will vendor-prefixed properties cause validation errors?
Vendor-prefixed properties are flagged as warnings, not errors. The validator recognizes them but recommends checking whether the standard unprefixed version is now supported.
Can I validate Sass or LESS files?
The validator checks standard CSS syntax. Sass (SCSS) and LESS must be compiled to CSS first. Validate the compiled output to check for issues in the final stylesheet that browsers will parse.