Returns child elements that have the given class name or tag name.

Please note the standard specifies this should return a live node list. This means, in Javascript for example, if you loop over the value returned by getElementsByTagName and getElementsByClassName and remove the elements, the length of the list will decrease. When I implemented this, I figured that was more trouble than it was worth and returned a plain array instead. By the time I had the infrastructure to make it simple, I didn't want to do the breaking change.

So these is incompatible with Javascript in the face of live dom mutation and will likely remain so.