Steps to reproduce:
After the work for simple attributes have been done (see https://bugzilla.mozilla.org/show_bug.cgi?id=1628418), the next step would be to implement IDREF element reflection.
HTML PR spec: https://github.com/whatwg/html/pull/3917
WPT test: https://github.com/web-platform-tests/wpt/blob/master/dom/nodes/aria-element-reflection.tentative.html
JFYI, Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=981423) and WebKit (https://bugs.webkit.org/show_bug.cgi?id=196843) already have an implementation.
https://github.com/webcompat/web-bugs/issues/112375 is covered by bug 1785412, not this one. Note that this part of ARIA reflection is implemented, but not shipped yet; see bug 1785412 for the reasons.