Checks CSS syntax and structure with line:column positions: unclosed or stray braces, unterminated strings, comments and url(), rules without a block, declarations missing a colon or value, misplaced !important, @charset/@import placement — plus hygiene warnings for duplicate properties in a block, unknown units and invalid hex colours. Strings, url() and nested at-rules are tokenised correctly. It is a syntax check, not a full property-value validator, and nothing is applied to this page.