export type UnknownHTMLElementTagNameFunction = ( tagName : string ) => boolean ; const unknownHTMLElementCache = { } ; export const isUnknownHTMLElementTagName = ( tagName ) => { const cache = unknownHTMLElementCache [ tagName ] ; if ( cache !== undefined ) return cache ; unknownHTMLElementCache [ tagName ] = global . document . createElement ( tagName ) instanceof global . window . HTMLUnknownElement ; return ! ! unknownHTMLElementCache [ tagName ] ; } ;