The special property of a DAG is that there are no cyclic paths between nodes. In the DOM, every node has a path to every other node, so you can create infinite loops (e.g. element.parentNode.nextElementSibling.previousElementSibling.firstChild).

That's the document tree - when it comes to application state, which is just data in memory, anything is possible. These state libraries tend to treat in-memory application state as the "real" state, and try to treat the document tree as a side effect. This is one reason that front end unit tests tend to miss bugs - application state being what you expected doesn't mean the user sees what they expected.