https://drafts.csswg.org/css-rhythm/#block-step-size
<rdar://problem/105519556>
Pull request: https://github.com/WebKit/WebKit/pull/10468
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/38628
Committed 261065@main (423f7933224a): <https://commits.webkit.org/261065@main> Reviewed commits have been landed. Closing PR #10468 and removing active labels.