Interestingly they don't seem to have a rule against one line declarations.

I alwats use this style, which imho is very handy, because of the tree structure and admittedly because I have a super cool macro in Vim that copies the characters from the line above, word by word so create rules that afect children of the rule above it requires just a few keystrokes:

  #some-div { margin:1em 0; }
  #some-div .inner { padding:5px 10px; }
  #some-div .inner p { font-size:90%; }

This makes the structure of the declarations more obvious imho, and I tend to have a nicely organized series of structures like that that are logically grouped together.

Obviously this applies more to components/widgets than the basic rules and layout.

If a declaration is long then I use newlines.. but even then I tend to group things together eg.

  #some-div {
    display:inline; margin: ...; padding: ...;
    border-radius: ...; border-color: ...; 
    font-size:90%; text-align:center;
  }

Basically within a css rule I'll group together the layout properties, the text properties, etc.