\Dom\DocumentFragment::getElementById() is not supported

Description

The following code:

<?php
$d = \Dom\HTMLDocument::createFromString('<p id="a">b</p><p id="a">c</p>');
$df = $d->createDocumentFragment();
$df->getElementById('a');

Resulted in this output:

  Error  Call to undefined method Dom\DocumentFragment::getElementById().

But I expected this output instead:

(no output)

This method is present in the DOM spec: https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment/getElementById and the similar method ::querySelector is implemented; I can't tell if querySelector actually uses the ID index though or does a linear scan through the fragment. (Aka, this method should not just be implemented, it is expected to have O(1) performance.)

PHP Version

PHP 8.4.13 (cli) (built: Oct  1 2025 20:34:15) (NTS)
Copyright (c) The PHP Group
Built by Debian
Zend Engine v4.4.13, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.13, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04.1

Start by reviewing the linked pull request and the existing Dom\DocumentFragment::querySelector implementation mentioned in the issue. Consult the DOM specification for getElementById() semantics, including duplicate IDs and the expected lookup behavior; done means the method is available and returns the matching fragment element as specified.