Question about implementation: while you are not building a browser (so I expect HTML is out of your scope) are you able to use an existing testsuite to compare your implementation?
We have our own test suite (orginally derived from the test suite of Meta's Yoga layout library [0]) which consists of text fixtures that are small HTML snippets [1] and a test harness [2] that turns those into runnable tests, utilising headless chrome both to parse the HTML and to generate the assertions based on the layout that Chrome renders (so we are effectively comparing our implementation against Chrome). We currently have 686 generated tests (covering both Flexbox and CSS Grid).
We would like to run the Web Platform Test suite [3] against Taffy, however these are not in a standard format and many of the tests require JavaScript so we are not currently able to do that.
[0]: https://github.com/facebook/yoga
[1]: https://github.com/DioxusLabs/taffy/tree/main/test_fixtures
[2]: https://github.com/DioxusLabs/taffy/tree/main/scripts/gentes...
[3]: https://github.com/web-platform-tests/wpt/tree/master/css/cs...