Looks like the fast-path is accepting the "n." form since it relies on  CharactersToDouble for "tokenization", thus accepting a wider range of inputs than CSSTokenizer does.

Example: https://jsfiddle.net/10aumthz/

Spec: https://drafts.csswg.org/css-syntax/#consume-number
     (https://drafts.csswg.org/css-syntax/#number-token-diagram)