function sameOrigin (A, B) {
  // 1. If A and B are the same opaque origin, then return true.
  if (A.origin === B.origin && A.origin === 'null') {
    return true
  }

  // 2. If A and B are both tuple origins and their schemes,
  //    hosts, and port are identical, then return true.
  if (A.protocol === B.protocol && A.hostname === B.hostname && A.port === B.port) {
    return true
  }

  // 3. Return false.
  return false
}
@link {https://html.spec.whatwg.org/multipage/origin.html#same-origin}
@param {URL} A
@param {URL} B