Vector and MonoBook add mediawiki.skinning.interface
module that includes h3 { padding-top: 0.5em; }
, while some skins do not. As a result, weird appearance is created. See the below list.
...?useskin=Vector m...?useskin=MinervaNeue ...?useskin=Modern ...?useskin=MonoBook ...?useskin=Timeless
So I propose test and change described in Special:Diff/3347620/3413921.
Thanks.