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              <h1 class="one">Header sized with em</h1>

<h1 class="two">Header sized with rem</h1>

<p>Try and increase the font-sizes of both of the headings. The padding of the first one gets larger and larger (since it's relative to the heading), while at the second heading it remains the same.</p>
              h1 {
  background: green;

.one {
  font-size: 3em;
  padding-left: .5em;

.two {
  font-size: 3rem;
  padding-left: .5rem;
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