This feature adds a dictionary as a second argument for document.createElement. The dictionary 'is' member is used to set custom element attribute. For now both syntaxes will be supported. Later on after v1 custom elements (https://www.chromestatus.com/features/4696261944934400) are shipped the string argument will be deprecated.
Docs: https://dom.spec.whatwg.org/#dom-document-createelement