The current algorithm definition requires adding two newlines:

3. A single SPACE (U+0020), followed by the string "{", i.e., LEFT CURLY BRACKET (U+007B), followed by a newline.
4. The result of performing serialize a CSS rule on each rule in the rule’s cssRules list, filtering out empty strings, indenting each item with two spaces, all joined with newline.
5. A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)

Based on WPT tests and the original commit of the current version, I think this is an obvious bugfix.