Last modified: 2014-10-06 07:16:14 UTC
The h3s in monobook are bolder than and subsequently can appear a bit bigger than h2s, despite being less important than h2s. Headers should appear with decreasing apparent importance according to which header they are.
Apparently this has mostly been fixed, and now really only affects some linux distros, where the h3 is essentially the same size as the h2, but darker. The css itself is in the mediawiki.skinning module in core, not in monobook specifically, but I'm not sure which component that would fit under.
Change 164713 had a related patch set uploaded by Isarra: Adjust h3 size to keep it smaller than h2 across normal platforms https://gerrit.wikimedia.org/r/164713
Change 164713 merged by jenkins-bot: Adjust h3 size to keep it smaller than h2 across normal platforms https://gerrit.wikimedia.org/r/164713