Currently we return nullptr in FakeStyleContext::GetStyleIfVisited used for Stylo values via Gecko_CalcStyleDifference.
I'm working on some patches that should handle this in bug 1380133.
Worth making sure this plays well with bug 1367904, which changes around visited stuff a little bit and is generally a big landing.
Yeah, I'm pretty sure I'll have to do some rebasing on top of that bug, so I will wait until I've done that before putting patches up for review.
Looks like :heycam's changes in bug 1380133 do indeed handle this for visited, IIUC.