Even if not implemented in such a sophisticated manner, "meaningful availability" is a better metric than pure uptime/downtime for most websites.

At one startup we worked at we had availability problems for some time, with the service going down in a semi-predictable manner ~2 times a day (and the proper bugfix a few weeks away). Because once a day the service went down was in the middle of the night with no one on call, pure availability was 80-90%. Given that it was a single country app with no one trying to do any business during the night, meaningful availability was ~99%. Knowing that gave us peace of mind and made tackling the problem a much more relaxed ordeal than the crunch time for a few weeks I've seen at other companies in similar situations.