HtmlTree.EMPTY is a special `Text` instance that is empty yet is not discarded when added to an `HtmlTree` via `add(Content)`. This is done via object identity check. For other content objects, the decision is made by invoking `Content.isValid()`. It would be preferable to override `isValid()` to return true in HtmlTree.EMPTY rather than have a special case for HtmlTree.EMPTY.