In order for `after.previousSibling` to be null, you'd have to have an element with a single text node and nothing else in it and we're attempting to insert an element before that text node. The way the comparisons work, though, is that the main loop would compare the new element with the text node. The new element wins, so the text node gets deleted and the new element inserted as the lastChild. Then the text node shows up in the new tree and the text gets inserted as lastChild.

So, we don't actually trigger any condition like that.