Slots allow to show elements from light DOM in specified places of shadow DOM. Try checking for existing elements in connectedCalback or by listening for slotchange events dispatched by your slot element ([HOST]Root. 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. . The slot is rendered correctly and everything appears to behave, except when flicking back and forth, once the earlier slot has been rendered. 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). There are two kinds of slots: Named slots: