Too Long; Didn't Read
Rem and em units in CSS create confusion for beginners. Em is relative to the font size of the parent of an element and rem is relative. Rem is only relative to its direct or nearest parent. Em becomes a trouble when you don't understand the relation between parents and child. Some people like to use rem units for consistency and scalability, while others like em units for places where the influence of nearby parent elements would make sense more than rem. Compounding effect of em is the power of compounding.