What is an “em” if the font-size of the document is specified in ems?

Question

In CSS, an em is a relative unit based on the font-size of the document. So, what exactly is an em then, if the font-size of the document itse

Answer 1

It will take the browsers' default value of 16px if no parent element has defined it.

Also see this: http://pxtoem.com/