Project: selector
Task (moderate): - append to `.root` with:
  `<div id="id17">
    <h1>H1</h1>
    <x>
      <h1>H1</h1>
      <x>
        <h1>H1</h1>
        <x>
          <h1>H1</h1>
        </x>
      </x>
    </x>
  </div>`
- `x` is one of section, article, aside, nav
- modify css for <h1> in `#id17`:
  - the first level (direct child) <h1> `font-size` is `30px`
  - the second level <h1> `font-size` is `25px`
  - the third and more level <h1> `font-size` is `20px`