Adrian Sandu explains some of the history of techniques using em and rem units in CSS, and shows some practical uses for the rem unit. More about rem. And default font-size turns out 16px. Source: Understanding and Using rem Units in CSS

Gajendar Singh explains and demonstrates the different length units available in CSS, including absolute, relative, and viewport-relative units. Turns out there are many length units. Although not all recommended for development. Also discover the mystery behind em and getting to know with rem. Source: A Look at Length Units in CSS

A quick run down on what viewport units are and how to use them to their full potential. First time acknowledge viewport units is when researching about WordPress responsive images. This adds more knowledge and shows how powerful viewport is. Source: Simplify Your Stylesheets With The Magical CSS Viewport Units | Tutorialzine