Using em works fine for margin and padding as it is based on the font size of the element itself. On the other
hand, it is not recommended to use em for font size because that is based on the font size of the parent which
leads to a compounding effect that is better off being avoided. A better, recommended approach for font size is
to use rem, i.e. root em, which as the name implies, is based on the font size of the root, i.e. 16px by
default.
For line-height, it is recommended to use unitless values, as it makes it based on the font size of the
root/browser, unless the element itself has a font size set. A good recommended value for line-height is 1.5.
React Basics
I reviewed components and composing them of other components.
I also looked at className for styling.
In addition, I looked at organizing components into separate files using export and import.