"Why this is on MDN?" It is taken from the spec[0]:

> The tabindex attribute must not be specified on dialog elements.

Why is the spec like that? Uhh, I cannot find a source or explanation. If I had to GUESS, it is because it uses autofocus instead of tabindex to establish focus to the dialog when it is shown.

[0]https://html.spec.whatwg.org/multipage/interactive-elements....

Here's a long conversation on it from 2016: https://github.com/whatwg/html/issues/1929

It does seem to have something to do with interfering with automatic focus on the dialog when it pops up as a modal.

The only reason dialog suddenly shipped is because browsers want to remove alert/popup/confirm.

Until last year the spec for dialog was in such a state that Chrome (the only ones implementing it) argued that it should be removed.

IIRC literally none of the issues were fixed but suddenly it got shipped in all browsers.