[Exposed=Window, LegacyUnenumerableNamedProperties]
interface HTMLAllCollection {
    readonly attribute unsigned long length;
    getter Element (unsigned long index);
    getter (HTMLCollection or Element)? namedItem(Utf16DOMString name);
    (HTMLCollection or Element)? item(optional Utf16DOMString nameOrIndex);

    // Note: HTMLAllCollection objects have a custom [[Call]] internal method and an [[IsHTMLDDA]] internal slot.
};