Since the markup-based mechanism for declaring custom elements has been dropped from the Custom Elements spec, the "Introduction to Web Components" should also drop all references and examples for the <element> element. Those should ideally be replaced with references and examples that match the script-based mechanism for defining custom elements in the current Custom Elements spec, but lacking that, in the short term the whole section should be dropped and replaced with note of some kind saying "Update to come." or something. Otherwise, the Intro doc is misleading and confusing readers.