feat: validate fixtures

[mfranzke]

Description

Resolves #38

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring

Testing

• Tests pass locally
• Added tests for new functionality
• Updated existing tests
• Manual testing completed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Browser Testing

If applicable, please test in the following browsers:

• Chrome
• Firefox
• Safari
• Edge

Performance Impact

If this change affects performance:

• I have run performance benchmarks
• Performance impact is documented
• Performance regression is acceptable for the added functionality

Breaking Changes

If this introduces breaking changes, please describe:

Additional Notes

Any additional information that reviewers should know.

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Latest commit: 91b2947

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[Copilot]

Pull Request Overview

This PR introduces a comprehensive fixture validation system for the CSS if() polyfill that uses Playwright to test CSS transformations in real browser environments. The system validates that polyfill-processed CSS produces the same visual results as expected output across multiple browsers and viewport sizes.

• Creates browser-based testing infrastructure using Playwright to validate CSS fixture pairs
• Adds automated validation for media queries, @supports queries, and computed style comparison
• Provides command-line tooling and documentation for running and managing fixture tests

Reviewed Changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/fixtures-validation/fixture-validation.test.js	Main test file implementing browser-based fixture validation with Playwright
test/fixtures-validation/fixture-validation.playwright.config.js	Playwright configuration for fixture validation tests
scripts/validate-fixtures.js	Command-line script for running fixture validation tests
package.json	Adds Playwright dependency and test:fixtures npm script
docs/FIXTURE_VALIDATION.md	Comprehensive documentation for the fixture validation system

Comments suppressed due to low confidence (1)

test/fixtures-validation/fixture-validation.test.js:31

• [nitpick] Function name '_getFixturePairs' uses a leading underscore which typically indicates private/internal functions, but this convention isn't consistently applied elsewhere in the file. Consider renaming to 'getFixturePairs' or applying the convention consistently.

const _getFixturePairs = () => {

[Copilot]

The function '_getFixturePairs' is defined but never used in the test file. Consider removing it or implementing its usage if it was intended for fixture discovery.