Pug mixin for an open Declarative Shadow DOM that polyfills itself.

shadow.pug

mixin shadow
	template(shadowrootmode="open")
		block
	script.
		{
			const element = document.currentScript.parentElement;
			if (!element.shadowRoot) {
				element.attachShadow({ mode: "open"})
				const template = element.querySelector(":scope > template[shadowrootmode=open]");
				if (template) element.shadowRoot.appendChild(template.content.cloneNode(true));
			}
		}