为什么自动保持边距的属性在水平工作时不能垂直工作？

I have seen that while developing websites, vertically centering a container (of fixed height) inside a container of random height always comes as a nightmare for the web developer (at least, me) while when it comes to horizontal centering a container (of fixed width) inside a container of random width, the I have seen that while developing websites, ver