The :nth-of-type() selector doesn't match in shadow roots. For example: #shadow-root <style> .item:nth-of-type(3) { ... } </style> <div class="item"></div> <div class="item"></div> <div class="item"></div> <div class="item"></div> Demo: Last 2 categories ("Men's T-Shirts" and "Ladies T-Shirts") on https://shop.polymer-project.org
*** This bug has been marked as a duplicate of bug 166748 ***