That's not true, first of all the root font size might not be immediate to query and find out what it is, second it might change because of variables on the root or base font size might be percentage/rem based itself not necessarily in pixels and finally setting it in pixels doesn't take into consideration zoom levels, or OS-level font settings.
> REMs have been around and standard practice for 10 years.
I know but thanks for flexing it out, in fact I'm arguing that css is and has been bad for many years