see https://www.w3.org/TR/CSS22/visudet.html#leading
Created attachment 417303 [details] Patch
Created attachment 417333 [details] Patch
Committed r271347: <https://trac.webkit.org/changeset/271347> All reviewed patches have been landed. Closing bug and clearing flags on attachment 417333 [details].
<rdar://problem/72962468>