All three engines support a misspelled alternate of aria-labelledby without the UK-English-style double l (aria-labeledby). I think this dates back to a WebKit implementation that Chromium inherited in the fork of Blink, and Gecko added later to match the others. The WPT test was added by Google in 2019 when Chrome shipped element reflection, and I've only recently become aware that this non-standardized behavior had consistent interop between the engines. Usage reports indicate the misspelling is low but not insignificant, so I and others think it should remain in the engines for the sake of web compat.

I'm filing this issue because I think the ARIA group should perform additional research (probably in the form of a WPT test PR) that fully fleshes out conflict resolution and other unknowns... For example, aria-labelledby should obviously win over aria-labeledby but should the misspelled aria-labeledby win over aria-label? We can use WPT to determine if the engines agree on that and other details.

Once those are determined, the ARIA WG will probably wish to document that engine behavior (as well as any needed conflict resolution in AccName), note the author error, etc.

[Update: 3 Jan 2024] Looking at @rahimabdi's results in Gecko (click "Show details"), Firefox's implementation of the non-standard spelling is either off or very incomplete (I have not checked Gecko source), so it's possible the ARIA WG decision may not be to document and keep it. I added the "Agenda" keyword to discuss either way.