Aha -- for `stretch` it looks like the fallback changed here (the old spec text said `Otherwise, this is equivalent to ''start''`): https://github.com/w3c/csswg-drafts/commit/1e436030985a2adbc9b54b5aade1ff882c5ed85d#diff-0b7c7fab23fb5f18a5f7ae5a9ec43fc8e9e4f1ac5f8ee16a02cb7d1ae2360e95L218
Bug 1543267 Comment 7 Edit History
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Aha -- for `stretch` it looks like the specced fallback behavior changed here (the old spec text said `Otherwise, this is equivalent to ''start''`): https://github.com/w3c/csswg-drafts/commit/1e436030985a2adbc9b54b5aade1ff882c5ed85d#diff-0b7c7fab23fb5f18a5f7ae5a9ec43fc8e9e4f1ac5f8ee16a02cb7d1ae2360e95L218