Comment 46 for bug 168164

This isn't entirely correct: " which requires that everything in the actual drawing content is based on SVG user units (px) only.", but ~suv is correct that the problem lies with the fact that Inkscape simply assumes 90dpi without fixing a viewBox and width/height. I also ran into this recently when converting from Inkscape to Corel Draw and this is an issue that really needs to be sorted out.

In other words: as far as I can tell it's not the fault of the other programs, we're simply outputting "undefined" SVG (always). As a quick fix it would probably be best to indeed output all (font) sizes as px, to ensure the least amount of "surprise". Once we fix Inkscape's horror of a unit system we can then reenable outputting sizes in other units.

As for the "em" unit, I would advise against making it available as a unit for setting the font size, as it is defined in terms of the current font-size. (Technically it is probably okay, but you then depend on inheritance of the font-size and/or the default font size of the program, which probably is not what 99% of the users want/expect.)