This is not possible with the current CSS statements you are talking about nth-whatever ,
However, this can be done using JavaScript ... unless, of course, you want to go down this route, the best way to do this would be with <span> tags, since then you will not have problems with people who disabled JS.
It is entirely up to you, but if I were you, I would just make friends and use JS, this is called progressive improvement and unobtrusive JS, if the content is not destroyed, if the user disables JS, this is acceptable, see here:
http://dowebsitesneedtobeexperiencedexactlythesameineverybrowser.com/
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