I think in terms of legibility, multi-lining comma separated selectors is much more readable. Could this be the default?

Example of desired effect:

.ball, 
.rabbit,
.wrapper:hover .contents,
.wrapper:active .contents {
  color: red;
}

What prettier currently emits:

.ball, .rabbit, .wrapper:hover .contents, .wrapper:active .contents {
  color: red;
}

The single line approach usually ends up creating really long lines that require horizontal scrolling and break n characters per line conventions (usually 80 or 120).