W3Schools example defines p:nth-child(2), which is not supported by the current versions of PD4ML. If you change it, for an instance, to p:nth-child(odd) – it renders correctly.

Also I added your style to my test document and it caused no CSS exception.

I used PD4ML v385fx2. So I would recommend to give a try to the newest version (and accompanying ss_css2.jar) – there are fully functional trials available for download.