Node.parentElement should be Element, not HTMLElement

[jonahShaf]

In lib.dom.d.ts Node.parentElement is defined as being HTMLelement | null. However, it should Element | null. The spec confirms this. This issue was originally fixed by #885 but was reverted in c81fa76. I didn't see any reasoning attached to that revert commit and the typing is still incorrect after the revert.

[HolgerJeromin]

This whole topic is not about implementing the spec but about usability.
#1151 (comment)