Sven, If you agree to have two methods: getListCSSClass and getLabelCSSClass (which apply respectively on li and span), and mark getCSSClass as deprecated (until marked as private), then the path is ready for branch wicket-1.5.x. I am waiting for your go-ahead to send the patch somewhere or submit the pull request on github.
If you do not agree, please tell me what I can do. Thanks & best regards, Sebastien.
