Project: selector
Task (moderate): - modify css for <h1> in `#id18`:
  - the first level (direct child) <h1> `text-indent` is `0`
  - the second level <h1> `text-indent` is `10px`
  - the third level <h1> `text-indent` is `20px`
  - the forth and deeper level <h1> `text-indent` is `30px`