Why isn't getElementById() available on Elements?

Question

Most DOM query methods are available on both Documents and Elements. For example,

console.assert(
  document.getElementsByTagName   && document.body.getElementsByTagName &&
  document.getElementsByClassName && document.body.getElementsByClassName &&
  document.querySelector          && document.body.querySelector &&
  document.querySelectorAll       && document.body.querySelectorAll
);

However, getElementById is only available on Document:

console.assert(document.getElementById);
console.assert(document.body.getElementById == undefined);

Why is this so?

---

The WHATWG DOM Living Standard tells us that:

Web compatibility prevents the getElementById() method from being exposed on elements

The W3C DOM4 Recommendation is a bit more specific:

The getElementById() method is not on elements for compatibility with older versions of jQuery. If a time comes where that version of jQuery has disappeared, we might be able to support it.

However, I still have a hard time understanding what the issue could be. How could the presence of this methods adversely affect the behaviour of jQuery or other libraries?

I've tried looking through old versions of jQuery (such as 1.0.0 and 1.7.0) to see if any of their use of getElementById hints at why this may have been a problem. I see that getElementById used to be buggy in some older browsers, but they detect that and fall back to a reliable manual implementation instead. I don't see anywhere that it could be called on an element and cause a bug. Where does this compatibility concern come from?

Answer 1

The git blame on https://github.com/w3c/dom’s master branch points to:

commit f71d7de304e1ee25573279157dd6ce1c2aa2c4f2
Author: Anne van Kesteren
AuthorDate: Tue Nov 26 13:53:41 2013 +0000
Commit: Anne van Kesteren <[email protected]>
CommitDate: Tue Nov 26 13:53:41 2013 +0000

Remove getElementById from Element. https://www.w3.org/Bugs/Public/show_bug.cgi?id=23860

and the linked bug describes how jQuery 1.2.5 + 1.2.6 (1.2.x?) are affected:

jQuery 1.2.5 assumes that any node it found in the DOM that has a "getElementById" property is a Document node. See https://bugzilla.mozilla.org/show_bug.cgi?id=933193#c17