I reject that claim as-is. In what situations would it break? Standards evolve. Changes to cookies, iframes, newly required HTTP headers all "broke" the web. Not to mention Flash deprecation. But somehow we survived.

Sure, this would theoretically be a backwards incompatible change. But if no one is using insertBefore without really meaning moveBefore, it's not a concern in practice.

You're both denying it would break, and saying it's okay for it to break, which are contradictory arguments.

Putting it your way, I'm saying it might break short-term, but long-term it would be fine.