timdown
appendChild not aware of void elements
At present, calling insertNodeAtPosition where n is a void element (e.g., <br>) causes the element to be appended as a child element, which results (potentially) in invalid nested elements.
I have added an array containing all the void elements as per http://www.w3.org/TR/html-markup/syntax.html#void-elements, and a function canAppendChild(element) to avoid this issue.
I like this conceptually, but I'm not sure whether it is the correct thing to do. Current versions of all the major browsers allow you to add child nodes to a <br> element via JavaScript, both directly and using Range.insertNode. If you do so, the content shows up in the childNodes property of the <br> element but is not rendered and does not show up in innerHTML. I would guess this behaviour is specified somewhere since all the browsers do it. I'll try to find out.
You are correct, elements can be appended to void elements, however they are not rendered. If changing existing functionality is undesirable, maybe just expose a method for checking whether an element can be appended as a child, that would satisfy my requirements also.