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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*.
https://dvcs.w3.org/hg/csswg/diff/7e2f406d89ab/cssom/cssom-source thanks jinks!