<h4>數位單位</h4>
<div>em: <span class="em">hello world</span></div>
<div>px: <span class="px">hello world</span></div>
<h4>物理單位</h4>
 <div>pt: <span class="pt">hello world</span></div>
 <div>cm: <span class="cm">hello world</span></div>
 <div>in: <span class="in">hello world</span></div>

.em {
 font-size: 1em;
 letter-spacing: 1em;
}
.px {
 font-size: 1px;
 letter-spacing: 1px;
}
.pt {
 font-size: 1pt;
 letter-spacing: 1pt;
}

.cm {
 font-size: 0.1cm;
 letter-spacing: 0.1cm;
}

.in {
 font-size: 0.1in;
 letter-spacing: 0.1in;
}

div {
 font-family: monospace;
 span {
  font-family: sans-serif;
 }
}