Const
see https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name see https://github.com/mathiasbynens/mothereff.in/blob/master/custom-element-name/vendor/is-potential-custom-element-name.js
see https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name see https://github.com/mathiasbynens/mothereff.in/blob/master/custom-element-name/vendor/is-potential-custom-element-name.js