See spec changes at https://github.com/whatwg/dom/pull/1079 for: - createAttribute() - setAttribute() - toggleAttribute() - createElement() - createElementNS() - createDocument() - createAttributeNS() - setAttributeNS() and https://github.com/whatwg/html/pull/7991 for updates to "valid custom element name" which impacts: - customElements.define() - customElements.whenDefined() - Whether elements are HTMLElement or HTMLUnknownElement ( https://html.spec.whatwg.org/#elements-in-the-dom:valid-custom-element-name )
<rdar://problem/95205349>