On Mac, caption rendering styles are not quite equivalent to what is previewed in the AX panel.
Created attachment 303184 [details] Patch
Created attachment 303185 [details] Patch
Comment on attachment 303185 [details] Patch Clearing flags on attachment: 303185 Committed r213285: <http://trac.webkit.org/changeset/213285>
All reviewed patches have been landed. Closing bug.