See test case here: http://hallvord.com/temp/moz/bug/wc412.htm Also attached. Firefox gets it right (per spec AFAIK), WebKit/Blink doesn't.
This was fixed in Chromium https://bugs.chromium.org/p/chromium/issues/detail?id=431915 Edge, Chrome and Firefox behaves now the same. Only WebKit is not behaving like the other browsers.
WebKit now handles this properly, too.