Add the actual DOM spec

[sideshowbarker]

https://dom.spec.whatwg.org/ is the only DOM spec now being actively maintained—the one that implementors are actually working from and the only one that’s incorporating new features and receiving bug fixes. (https://www.w3.org/TR/dom/ is a frozen Rec that’s not up to date with current implementations and so should not be used by implementors or web developers, and nobody is working on WD to replace it.)