get templateElement() {
      if (this.firstElementChild === null) {
        const template = this.ownerDocument.createElement("template");
        this.appendChild(template);
        return template;
      } else if (this.firstElementChild instanceof HTMLTemplateElement) {
        return this.firstElementChild;
      }
      this.#raise("first child element must be a <template> element");
    }
Gets the main `<template>` used for rendering