The example below shows how relative font-units scale proportionally in relation to their parent. The following HTML and CSS is used:

div.someClass { font-size: 10px; }
div.someOtherClass { font-size: 15px; }
p.theP { font-size: 2em; }
<div class="someClass">
	<p class="theP">Some text</p>
</div>

<div class="someOtherClass">
	<p class="theP">Some other text</p>
</div>

Paragraph 1:

Some text

Paragraph 2:

Some other text