function getDoctype() {
  // An example of this is warnerbros.com/archive/spacejam/movie/jam.htm
  if (!document.doctype) {
    return null;
  }

  const documentCompatMode = document.compatMode;
  const {name, publicId, systemId} = document.doctype;
  return {name, publicId, systemId, documentCompatMode};
}
Get and return `name`, `publicId`, `systemId` from
`document.doctype`
and `compatMode` from `document` to check `quirks-mode`
@return {{name: string, publicId: string, systemId: string, documentCompatMode: string} | null}