This is not used. align-content uses StyleContentAlignmentData which is more expressive.
Pull request: https://github.com/WebKit/WebKit/pull/8242
Committed 258509@main (19d6f99587d7): <https://commits.webkit.org/258509@main> Reviewed commits have been landed. Closing PR #8242 and removing active labels.
<rdar://problem/103927233>