CodePen Embed - css-em

<div class="box">
  <span class="title1">title 1</span>
  <span class="title2">title 2</span>
</div>

html{
  /*  Chrome默认字体大小为16px  */
  font-size: 2rem;
}

.title2{
  /*  其他元素相对于根元素，所以是32*2=64px  */
  font-size: 2rem;
}

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