Thanks [I'm the author]. I tested with Chrome 105 on macOS and it succeeded. Possibly there are OS/plugin/etc issues?

Of course, I know there is no guarantee that every browsers innerHTML implementation will produce exactly the same result, but so far I haven't found any variation (Chrome, FF, Safari, Edge).

In Firefox 105.0.1 on MacOS, the button also always fails when I click it.

EDIT: In my case, it appears to be some extra "<div style=\"position: static !important;\"></div>" text added before the closing </body> tag. I suspect this is introduced by a plugin, probably LastPass.

Fails on Safari on iPadOS 15.6.1 too.

edit: another commenter says ad blockers are the culprit.

It also fails on Firefox 104 on Linux.

It might be a browser extension; on my Firefox install I have browser extensions that add HTML to every webpage, making it fail.

Html wouldn't be a Web standard if it were consistent across browsers.