Allow for pseudo-class position with repeat > 9n, no additional offset by cjwillcock · Pull Request #13 · ThomasWeinert/PhpCss (GitHub)
I found with selectors that include a pseudo-class with position, such as :nth-child(10n), the parser generates a token mismatch if the NUMBER is more than one digit. I traced this Scanner/Patterns...
I have found the PhpCss library from Thomas Weinert very useful. I used it so much, I found a bug and fixed it. It’s nice that this library has unit tests so I could quickly create a new pattern and expected output, then make my little correction.

Typical programming mistake, a missing plus (+)!