Add node data chainables.
Add skeleton for DOM client classes.
Add Chainable.forEach method.
Implement node creation and basic readonly properties.
Implement Node.insertBefore.
Implement Node.removeChild.
Implement Node.replaceChild.
Implement element's child / sibling element references.
Implement element attribute methods.
Proper undefined protection.
Implement user data.
Implement CharacterData and Text interfaces.
Cover ProcessingInstruction interface in tests.
Implement Element.attributes.