Add parent-first-child top and parent-last-child bottom margin collapsing using a two-level approach (Level A shifts child inside parent, Level B adjusts grandparent for effective collapsed margin). Fix pre-existing bug in shift_subtree_x that incorrectly skipped absolute/fixed children. Extract shared MARGIN_EPSILON constant, add 9 unit tests and 4 golden tests (209-212), promote 31 WPT tests to pass. Includes code review fixes: pub(crate) visibility for is_root_element, known-limitation docs, and inline-content shift test coverage. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
378 B
378 B