Support external CSS

[dfop02]

Description

Add External CSS Support

Adds support for <style> tags and loading CSS from external files via <link rel="stylesheet"> tags.

Features

• Local and remote CSS files: Supports both local file paths and HTTP/HTTPS URLs
• Selective parsing: Only loads CSS rules relevant to HTML elements, improving performance with large CSS frameworks
• Multiple CSS files: Can load multiple external stylesheets
• Combined with inline styles: Works alongside <style> tags and inline styles
• Error handling: Gracefully handles missing or invalid CSS files without breaking HTML conversion

Usage

<link rel="stylesheet" href="https://example.com/style.css">
<link rel="stylesheet" href="./local/styles.css">

External CSS rules follow the same precedence as <style> tag rules and are applied according to standard CSS specificity.

Implementation

• Added fetch_external_css() utility function for loading CSS from URLs and local files
• Extended HTML parser to detect and process <link> tags
• Integrated with existing CSS parser using selective parsing mode
• <link> tags are automatically removed from the output document

Issue Reference

None

Checklist Before Requesting a Review

• I have performed a self-review of my code.
• My code follows the project's coding style and guidelines.
• I have run tests and verified that all existing and new tests pass.
• I have added new tests to cover my changes.