On Feb 12, 2007, at 10:51 AM, jiho wrote:


OK, thanks for this detailed and very clear answer. so 72ppi is a  

kind of legacy in fact? and then 90ppi is just an arbitrary choice  

which reflects more today's hardware? (arbitrary is ok for me, it's  

just nice to understand things completely)



72 DPI is legacy from when Mac's and such brought in desktop publishing and replaced the traditional 72.27 DPI for points.

90 DPI is the recommended reference case for the "px" unit that is specified in the CSS spec. The SVG spec in turn calls out the CSS spec and it's "px" units.

So the 90 DPI base assumption is in compliance with SVG. Getting a value from the actual display would also be in compliance, but does take quite a bit more work and provides quite a few more problems.