convert the Word val to a CSS unit
        val     : The raw Word val
        factor  : The conversion factor. If font sizes, typically factor=1/2., others factor=1/20.
        unit    : The CSS unit to which we are converting, default CSS.rem
        pt_per_em : The number of CSS.pt per em. 12. is the default, but 'tain't necessarily so.

def val_to_css(C, val, factor, unit=CSS.rem, pt_per_em=12., decimals=2):
        """convert the Word val to a CSS unit
        val     : The raw Word val
        factor  : The conversion factor. If font sizes, typically factor=1/2., others factor=1/20.
        unit    : The CSS unit to which we are converting, default CSS.rem
        pt_per_em : The number of CSS.pt per em. 12. is the default, but 'tain't necessarily so.
        """
        return (round(float(val) * factor / pt_per_em, decimals) * CSS.rem).asUnit(unit)