astro/packages/integrations/lit/client-shim.js

async function polyfill() {
	const { hydrateShadowRoots } = await import(
		'@webcomponents/template-shadowroot/template-shadowroot.js'
	);
	hydrateShadowRoots(document.body);
}

const polyfillCheckEl = new DOMParser()
	.parseFromString(`<p><template shadowroot="open"></template></p>`, 'text/html', {
		includeShadowRoots: true,
	})
	.querySelector('p');

if (!polyfillCheckEl || !polyfillCheckEl.shadowRoot) {
	polyfill();
}