Using REM instead of PX makes your typography responsive. Here is why and how.
Why REM?
REM is relative to the root font size. When users change their browser font size, REM-based layouts adapt automatically.
Quick Reference
With a base of 16px: 12px = 0.75rem, 14px = 0.875rem, 16px = 1rem, 18px = 1.125rem, 20px = 1.25rem, 24px = 1.5rem.