content の count(...) で挿入するカウンタをカウントアップします。

body {
  counter-reset: chapter;
}
h1 {
  counter-increment: chapter;
  counter-reset: section;
}
h1:before {
  content: counter(chapter) ". ";
}
h2 {
  counter-increment: section;
  counter-reset: subsection;
}
h2:before {
  content: counter(chapter) "." counter(section) " ";
}
h3 {
  counter-increment: subsection;
}
h3:before {
  content: counter(chapter) "." counter(section) "." counter(subsection) " ";
}

<h1>Web入門</h1>
<h2>HTML</h2>
<h3>HTMLとは</h3>
<h3>HTMLリファレンス</h3>
<h2>CSS</h2>
<h3>CSSとは</h3>

• https://www.w3.org/TR/CSS2/generate.html#propdef-counter-increment
• http://www.y-adagio.com/public/standards/tr_css2/generate.html#propdef-counter-increment
• https://developer.mozilla.org/ja/docs/Web/CSS/counter-increment
• http://caniuse.com/#feat=css-counters