Let's start this chapter by reviewing the basics of Reboot when using content components in Bootstrap. One of the main changes for content components in Bootstrap 4 is the switch from em to rem units of measure. rem is short for root em and is a little bit different from a regular em unit. em is a relative unit to the font-size of the parent element it is contained within. This can cause a compounding issue in your code that can be difficult to deal with when you have a highly nested set of selectors. The rem unit is not relative to its parent, it is relative to the root or HTML element. This makes it much easier to determine the actual size of text or other content components that will appear on the screen.
The box-sizing property is globally set to border-box on every element. This is good because it ensures that the declared width of an element doesn't exceed its size due to excess margins or padding.
The base font-size for Bootstrap 4 is 16px and it is declared...