lib.dom.ts defines Node.parentElement as never-null:
https://github.com/Microsoft/TypeScript/blob/95c3eccbe9f3f7e1857ce157d437e6b80b3c9c6f/lib/lib.dom.d.ts#L8749

However, it's a pretty common pattern to write loops like

while (n.parentElement != null) {
  // [...]
  n = n.parentElement;
}

to walk up the node tree.

Also parentNode, previousSibling, nextSibling, firstChild, lastChild... (?)