Monobook makes frequent use of line-height specifications using the unit "em".
While these lengths _are_ font-size-relative, which is good, the computed value,
and not the relative value is inherited by child elements, which is bad.
The problem becomes apparent when a child node uses a different font-size. E.g.
many boxes on Wikipedia use font-size:80%. However, because the computed value
of line-height is inherited, the line-height stays unchanged (i.e. too large).
The solution is to use plain number line-heights, i.e. "1.5" instead of "1.5em".
That way, the line-height will properly scale when the font-size is changed.
Here's the relevant section from the CSS2 spec:
Values for this property have the following meanings: [...] <length> The box height is set to this length. Negative values are illegal. <number> The computed value of the property is this number multiplied by the element's font size. Negative values are illegal. However, the number, not the computed value, is inherited.