Performance: check out ParentNode.children instead of Node.childNodes

[matatk]

@BillyRayPreachersSon suggested I check out ParentNode.children instead of Node.childNodes so that I can remove the check for each node being visited being an element node. Perhaps the check for the existence of the element could be skipped too.

Note also that it's an HTMLCollection, so a normal for loop will need to be used (may well also speed up the code as currently it's a forEach function-calling loop).