rephrase:
"An visitor that produces a formatted string representation of the CSS tree"

Then move the comment about compact down to the visit method. for instance:
```dart
  /**
   * Visits the css [tree] to produce a formatted css string. Call [toString] to
   * get the formatted string. When [compact] is false, this visitor generates
   * a pretty format useful for user debugging.
   */
  voic visit(tree, [bool compact = false]) {