Document: createElement() method

A string that specifies the type of element to be created. The nodeName of the created element is initialized with the value of tagName. Don't use qualified names (like "html:a") with this method. When called on an HTML document, createElement() converts tagName to lower case before creating the element. In Firefox, Opera, and Chrome, createElement(null) works like createElement("null").

An object with the following properties:

The tag name of a custom element previously defined via customElements.define(). See Web component example for more details.