<html>
  <body>
    <div id="fa">
      父元素节点
      <div id="ch1">rem</div>
      <div id="ch2">em</div>
      <div id="ch3">em</div>
    <div>
  </body>
</html>
html {
  font-size: 10px;
}
#fa {
  font-size: 20px;
}

#ch1 {
  background: red;
  width: 2rem; /* 转换后 20px */
}

#ch2 {
  width: 2em; /* 转换后 40px */
  background: skyblue;
}

#ch3 {
  font-size: 15px;
  width: 2em;  /* 转换后 3s0px */
  background: pink;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.