Details

Properties within rule sets should each reside on their own line.

Bad

p {
  margin: 0; padding: 0;
}

Good

p {
  margin: 0;
  padding: 0;
}

A special exception is made for single line rule sets. For example the following is acceptable:

p { margin: 0; padding: 0; }

If you want to also report a lint for single line rule sets, set the allow_single_line_rule_sets option to false.