What is Em in CSS? When to use it?

Answer 1

em is a unit of measurement that is relative to the font-size of the parent. Many people get it confused with rem which is relative to the html root and excellent for resizing especially when you need to for mobility. In modern code, you can see these used over the standard pixel unit.

j.ermey.net has a good article about his choice to use em over rem and examples of how they work.

http://j.eremy.net/confused-about-rem-and-em/

In making your choice what units to use, I do suggest looking at the html framework you plan on using to make sure your CSS works properly in tandem.