I just wanted to login on chat.stackexchange.com, but couldn't. When I went to the help page, the checks failed on test 5. It claimed:
Your browser either does not support localStorage, or it is disabled for stackauth.com. Either upgrade your browser, or enable localStorage.
In fact, the problem was that Chrome blocks the third-party cookie for stackauth.com by default (it's in Dutch, but it just shows the "Blocked" cookies for the current page):
After allowing this cookie, the login works just fine. So IMHO it would really help to add some sort of hint in the test 5 error text that blocking cookies from stackauth.com might also cause the login failure and allowing that cookie could fix it.