While it sounds odd, according to the spec the following constraint needs to be satisfied: 'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom' = height of containing block
Created attachment 343888 [details] Patch
Committed r233348: <https://trac.webkit.org/changeset/233348>
<rdar://problem/41616586>