What is the effect of NodeList.item(-1) or Node.appendChild(null)

What is the effect of NodeList.item(-1) or Node.appendChild(null)?

Answer:

As mentioned in DOM Level 3, "DOM operations only raise exceptions in "exceptional" circumstances, [...] Implementations should raise other exceptions under other circumstances. For example, implementations should raise an implementation-dependent exception if a null argument is passed when null was not expected.", therefore if an attribute is declared unsigned, using a negative should generate an error. Note that the error itself is not defined and is therefore implementation and binding dependent.