The steps are described below:
1.Let the {preferred style sheet set name} be *current*.
2.Set {preferred style sheet set name} to *name*.
3.If *name* is not a case-sensitive match for *current* and {last style sheet set name} is null {enable a style sheet set} with name *new*.

I think: step 1 should exchange {preferred style sheet set name} and *current*, and the *new* in the end of step 3 should be *name.

Final description:
1.Let *current* be the {preferred style sheet set name}.
2.Set {preferred style sheet set name} to *name*.
3.If *name* is not a case-sensitive match for *current* and {last style sheet set name} is null {enable a style sheet set} with name *name*.