The rem unit

The rem relative unit of measurement sets a font size of the element depending on the font of the root element - <html>. For example, a font equal to 2rem means that it is twice the size of the root element.

Example

Let's set the heading text font one and a half times larger than the root element font, and 1rem to the paragraph:

html<h1>Text1</h1>
<p>text2</p>

cssbody{
  font-size: 14px;
}
h1 {
  font-size: 1.5rem;
}
p {
  font-size: 1em;
}

Результат:

See also