The base font-size is determined by the users pre-defined preferences within the browser.
In almost every browser, 16px is the standard for proportional fonts. This can also change dependant on if the font uses serifs or is a fixed width font.
Just remember, em is relative to the element it is used on or relative to the inherited parents font-size, and is proportional to that. rem however, uses the root html elements.
For example:
html {
font-size: 16px;
}
h1 {
font-size: 2em; // 32px
}
p {
font-size: 1em; // 16px
}
.someClass {
font-size: .75em; // 12px
}
.someClass p {
font-size: 2em; // 24px
}
.someClass p .test {
font-size: 1.25rem; // 20px
}<html>
<h1>2em Title Text</h1>
<p>Normal Element Text</p>
<div class="someClass">
someClass font size
<p>SomeClass with em</p>
<p><span class="test">someClass p element with class test</span>
</p>
</div>
</html>