stylesheet's relative versus absolute

am generating a web site and prefer the device-dependent sizes and for the
fonts and margins using 'px'.  as this changes with the screen, i like this.
however, the WCAG and Bobby recommend 'em' and percentages.  For example,
form WCAG, "Only use absolute length units when the physical characteristics
of the output medium are known, such as bitmap images." 

As 'px' is device-dependent, it is not absolute nor realtive.  Therefore, is
there an issue to use 'PX' fonts sizes and margins in Stylesheets?

thanks