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The rounding only kicks in after the scaling to 2048 UPM.
It was a tantalizing fantasy, for a moment, the notion that we were all laboring under an unnecessary limitation all these years.
I am unsure whether this capability was carried over to OpenType CFF (.otf); probably not.
I recall that there was an earlier version of Robofont that allowed editing with fractional coordinates, maybe when it was in beta?
Font file size doesn't seem to be the issue that it once was.
For high volume web sites, or web sites with lots of mobile users, font sizes matter.