This is a quirk that was introduce long time ago to adjust the vertical position of a <br> run (only in renderer tree output) so that it's as if the table cell content wasn't aligned vertically.
Created attachment 418749 [details] Patch
Created attachment 418760 [details] Patch
Created attachment 418798 [details] Patch
Created attachment 418801 [details] Patch
Committed r272100: <https://trac.webkit.org/changeset/272100> All reviewed patches have been landed. Closing bug and clearing flags on attachment 418801 [details].
<rdar://problem/73784392>