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'll be relative to whatever the browser's default font-size is, which WILL be specified in a physical size, e.g. '10pt'.