In skins/common/commonElements.css the
h1, h2, h3, h4, h5, h6 { width: auto; }
seams unnecessary, because auto is the initial value for width:
http://www.w3.org/TR/CSS2/visudet.html#propdef-width
In https://svn.wikimedia.org/viewvc/mediawiki?view=revision&revision=95837
the width:auto was copied from skins/vector/screen.css
but skins/monobook/main.css hadn't width:auto.
skins/vector/screen.css had the width:auto from the first revision:
https://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/skins/vector/screen.css?revision=51086&view=markup
Here is an unanswered question for the reason for width: auto:
https://bugzilla.wikimedia.org/show_bug.cgi?id=1629#c55
Please mark this bug as INVALID when there is a reason for width: auto.
Version: unspecified
Severity: enhancement