Looking up an element definition no longer depends on the element's node document

[annevk]

What is the issue with the HTML Standard?

As pointed out in https://issues.chromium.org/issues/347963247 we currently use the node document to determine an element's definition and this can cause some unexpected behavior when an element is defined in one document and then moved to another.

When an element always has a registry associated with it this can no longer happen. Mainly filing this to make sure that's properly tested.

cc @dbaron