So once in a lifetime or so, I might want to vertically-center the WordPress login.

Here’s the css:

html {
  height: 100%;
  margin: 0;
  padding: 0;
}
html body {
  display: table;
  margin: 0 auto;
  height: 100%;
}
body #login {
  padding: 0;
  display: table-cell;
  vertical-align: middle;
}

Enjoy!

(Read more about vertically-centering login forms.)