Current state of interface in MediaWiki includes big inconsistencies in sizing interface elements in extensions built on-top.

(Collection of button sizes within 15 minutes of English Wikipedia. Not everything is connected to the task, though some of the sizing portions are visible)

One of the reasons at core is that we're still relying on em based sizing – with its advantages like zoomability – but didn't go for one base font size as anchor for the sizing of elements with Vector as quasi-default skin in our main projects.
Instead we have to deal with above-mentioned inconsistencies with its negative effect on professional look & feel and user satisfaction.
There are also further side-effects, like

• problematic hand-over from designers visual templates to implementors
• slightly confusing inheritance of ems which lead to unwanted outcomes
• rounding errors in browsers, which need counter-intuitive hacks to address those.

This task is meant to collect the current shortcomings and evaluate possible solutions.