I believe that there are no specifications that one must use only British or American English writing style in CSS. However, it must be taken care of that whenever one writes anything one must use only one of the two formats. One can follow either American English or British English. That is to say; one must not be spelling "Center" as " center and centre" " American and British" at the same time. Follow only one of the two formats. So one must have idea about the spellings of words that are spelled in different ways in the two styles. In grammar, Americans use the simple past tense whereas British talk about the same time through the present perfect tense. I regard uniformity as the right way of writing be that American or English; not the combination of the two.