Yep, I think the priorities should be <html> and <body>. Here's mine, which centers the content both horizontally and vertically via flexbox:

    html {
      display: flex;
      align-items: center;
      justify-content: center;
      min-height: 100%;
    }

    @media screen {
      body {
        max-width: 40rem;
        margin: 1rem;
      }
    }

It's not perfect, but it works well for simple websites (e.g. https://colorclock.hashbase.io/).