Check out example codes for "line-height not working". It will help you in understanding the concepts better.
Code Example 1
Because the em tag is inline and its line-height cannot be lower than its parent div. For example, if you set the line-height of the parent to 10px, then you would be able to decrease the line-height of em tag to 10px as well.
Learn ReactJs, React Native from akashmittal.com