User blog comment:Kirkburn/Technical Update: October 26, 2015/@comment-5645428-20151027121859/@comment-126761-20151027140137

The scrolling behaviour of Special:CSS was particularly odd - depending on your screen size, you'd often end up having to scroll two different things in order to read/edit the whole thing. Very frustrating (and never intended!).

Regarding Common.css loading behaviour, this is covered in more depth here: Help:Community CSS and JS. This was originally done to avoid weird effects when Wikia switched to the current skin. We're very happy to re-enable Common.css loading on older communities, but we don't want to do it without being asked in case it breaks some customizations.