<ol>
<li >第一層
  <ol> 
    <li>第二層
      <ol>
        <li>第三層</li>
        <li>第三層</li>
        <li >第三層</li>
      </ol>
    </li>
    <li>第二層</li>
    <li>第二層</li>
  </ol>
</li>
<li>第一層
  <ol>
    <li>第二層</li>
    <li>第二層</li>
  </ol>
</li>
</ol>
ol{
  counter-reset:num;
  list-style-type: none;
}
li:before{
  content:counters(num, ".") ":";
  counter-increment:num; 
}

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