Try checking for existing elements in connectedCalback or by listening for slotchange events dispatched by your slot element ([HOST]Root. Note that when applying the restamp attribute to dom-if. As you can see, slot-based composition is a powerful tool that allows widgets to pull in the page content without cloning or modifying the DOM. To access children assigned to slots in your shadow root, you can use the standard [HOST]edNodes or [HOST]edElements methods with the slotchange event. . There are two kinds of slots: Named slots: