Comment 17 for bug 166970

Let me vote for that bug, as I think inkscape should not implicitly make a relative unit an absolute one. A default is very okay, let's say 90dpi or anything else. But the SVG-standard also emphasizes the "px" unit as "user unit", therefore relative and negotiable.

So by allowing absolute units (cm, in, em, pt...), SVG is indeed capable of designing "fixed"-sized elements, where sizes and zooms have to be interpretede by different software. mixing absolute/relative units may cause bad things to your SVGs when displaying them with different soft- and hardware.