Focus trap on native <dialog> element

[petermurphy-welldot]

We've recently had an accessibility audit, and the auditors are citing a 2.4.3 Focus Order failure arising from our use of modal <dialog> element (opened via showModal()). They are expecting focus to be trapped within the <dialog> element, and not be allowed to escape to the browser chrome. I shared with them the whatwg/html#8339 discussion, and they've conceded that this is not a WCAG failure, but still recommend that we implement a focus trap here as the expected behavior and the optimal screen-reader experience.

I know it is not required to meet WCAG conformance, but I'm also wondering if there is any positive reason not to implement a focus trap as far as WCAG is concerned.