Message boxes in shared.css currently feature inline-block to originally provide intrinsic width boxes, coming from https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/62439/ and T35438: SpecialPreferences: <p> should not be inside <strong>.

We needed to add a general, neutral messagebox treatment, but inline-block in there collided with same class used in mbox and votearchive templates as reported by @Perhelion in T232553#5506657.

Back in 2013, the box messages were also used in special pages like Special:Preferences. This is not or only scarcely true anymore. For a box message it's intended to be a block level element by definition and inline-block should be the exceptional override if at all.