In Github's case, I think it was also because a lot of security boundaries were using TLD which led x.github.com potentially grab cookies of y.github.com or wor

In Github's case, I think it was also because a lot of security boundaries were using TLD which led x.github.com potentially grab cookies of y.github.com or worse, github.com itslef https://news.ycombinator.com/item?id=5500612 https://news.ycombinator.com/item?id=5500612