code: public SafeStyleBuilder backgroundAttachmentAppend(String value, String... otherValues) {
      appendToProperty("background-attachment", sanitizeAndJoinEnumValues(value, otherValues));
      return this;
    }
docs: Appends {@code values} to the {@code background-attachment} property. Values will be comma
  separated.
  
  <p>The values must consist of only ASCII alphabetic or '-' characters. A non-conforming value
  will be replaced with {@link #INNOCUOUS_PROPERTY_STRING}.
  
  @see "https://developer.mozilla.org/en-US/docs/Web/CSS/background-attachment"