User blog comment:Kirkburn/Technical Update: July 1, 2015/@comment-4189499-20150703141222/@comment-452-20150704155139

Those reasons are also exactly why I think not showing the boundaries is a bad idea.

Conversely, I see no reason to display white on either side of the article preview.

BTW, I'm not putting it in my personal CSS, because I want to make sure everyone on the wiki knows where the article area ends. Now, Wikia can turn around and play the "adding a grey border around the preview violates the TOU because it's supposed to be white so you can't tell where the article end" card, but hopefully they're not that petty.