v2:
* use non-deduped seq for attrs; leave foreign attr adjustment to impl
  (I think caseTable can be removed too)
* createComment -> insertComment (so Handle can be set to ParentNode)
* zero-copy insertCharacter
  (maybe attr values too? but that needs a more elaborate CoW scheme)
* non-ref Token
* sink/lent in iface
* standardize identifier names
* omit openElementsInit, just add HTML based on ctx?
* allow custom seq?
etc:
* add getElementById and some more basic stuff to minidom
