If 2 out of 100 people I know see a broken website, depending on the website, that's fine, that doesn't sound like a big deal. Now, if out of 10 power users, all ten of them see a broken site once every 50 logins? Thats a much bigger deal. 98% can be more than enough or not remotely enough depending on the units involved but there are plenty of cases where it's fine to not support the last 2 internet explorer users and stuff