Last modified: 2013-04-22 16:17:17 UTC
As far as I know this is a relic for compatibility with IE<6, which couldn't handle borders properly. And according to https://www.mediawiki.org/wiki/Compatibility#Browser we don't even fully support IE6 these days.
Gerrit change #44825.
Merged (I0a1fed69) -> marking as fixed.