Otherwise, requesting a min size in em where the equivalent in px had a
fractional part would lead to the gadget getting allocated 1 too few px.
You could see this in the CSS property vs. allocation in the Inspector.

Note that margin/border/padding are left alone: the rationale is that we
do as browsers do, and Benjamin said we already do that for those,
whereas his tests on min-(width|height) showed otherwise.  My subsequent
analysis indicated it to be far less clear-cut than that, but he remains
unconvinced that we should ceil() all the things! So just do these ones.

GNOME/gtk#1088