A slot is a placeholder inside a web component that users can fill with their own markup (see Using templates and slots for more information). When working with Shadow DOM-enabled components, the element is a web platform native way to allow content projection into components. 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. Try checking for existing elements in connectedCalback or by listening for slotchange events dispatched by your slot element ([HOST]Root. Slots allow to show elements from light DOM in specified places of shadow DOM. Note that when applying the restamp attribute to dom-if. 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: