Project: selector
Task (moderate): - append to `.root` with:
  `<div id="id16">
    <h1>H1</h1>
    <section>
      <h1>H1</h1>
      <article>
        <h1>H1</h1>
        <aside>
          <h1>H1</h1>
          <nav>
            <h1>H1</h1>
          </nav>
        </aside>
      </article>
    </section>
  </div>`
- modify css for <h1> in `#id16`:
  - the first level (direct child) <h1> `font-size` is `30px`
  - the second level <h1> `font-size` is `25px`
  - the third level <h1> `font-size` is `20px`
  - the forth and deeper level <h1> `font-size` is `15px`