$ show premailer/css_parser
This repository is a Ruby gem for parsing CSS that implements CSS specification compliance including selector specificity calculation, property parsing, shorthand expansion/creation, and CSS cascade merging. It handles CSS imports, media queries, and URL resolution with built-in SSRF protection. The library processes CSS input (files, URIs, strings) into RuleSet objects that can be queried, manipulated, and merged according to W3C CSS standards. Key features include robust parsing of complex CSS, handling of edge cases in shorthand properties, and security measures against remote file inclusion attacks.
The LLM identified 3 files worth highlighting for reuse:
Implements W3C CSS specificity calculation algorithm. Takes a CSS selector string and returns its specificity value as defined in CSS2.1 specification. Dependencies are minimal (two regex constants for pseudo-classes/elements). Demonstrates correct implementation of a non-trivial web standard algorithm that could be extracted for use in any CSS processing tool.
Converts relative URLs in CSS to absolute URLs per CSS specification. Takes CSS string and base URI, resolves url() references while preserving already-absolute URLs. Dependencies: Addressable::URI and one regex constant. Implements URL resolution correctly per CSS2.1 spec, handling edge cases like quoted URLs and various URI schemes. This is a self-contained utility function that solves a common problem in CSS processing.
Technique for preserving whitespace inside CSS functions when tokenizing values. Replaces whitespace within function calls (e.g., calc(), var()) with placeholders before splitting on spaces, then restores the original whitespace. This handles edge cases like 'margin: calc(1em / 2 * var(--foo));' where spaces inside functions must be preserved. While currently a private method, this algorithm is a reusable pattern for parsing languages with nested function syntax where internal whitespace is significant.