Render Custom Element Tag (#2473)

* Support rendering web components

* nit: remove addition of script

packages/astro/src/runtime/server/index.ts

@@ -148,7 +148,7 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
 	}
 	const probableRendererNames = guessRenderers(metadata.componentUrl);
 
-	if (Array.isArray(renderers) && renderers.length === 0 && typeof Component !== 'string') {
+	if (Array.isArray(renderers) && renderers.length === 0 && typeof Component !== 'string' && !HTMLElement.isPrototypeOf(Component as object)) {
 		const message = `Unable to render ${metadata.displayName}!
 
 There are no \`renderers\` set in your \`astro.config.mjs\` file.
@@ -165,6 +165,12 @@ Did you mean to enable ${formatList(probableRendererNames.map((r) => '`' + r + '
 				break;
 			}
 		}
+
+		if (!renderer && HTMLElement.isPrototypeOf(Component as object)) {
+			const output = renderHTMLElement(result, Component as typeof HTMLElement, _props, slots);
+
+			return output;
+		}
 	} else {
 		// Attempt: use explicitly passed renderer name
 		if (metadata.hydrateArgs) {
@@ -434,6 +440,31 @@ export async function renderAstroComponent(component: InstanceType<typeof AstroC
 	return template;
 }
 
+export async function renderHTMLElement(result: SSRResult, constructor: typeof HTMLElement, props: any, children: any) {
+	const name = getHTMLElementName(constructor);
+
+	let attrHTML = '';
+
+	for (const attr in props) {
+		attrHTML += ` ${attr}="${toAttributeString(await props[attr])}"`;
+	}
+
+	children = await children;
+	children = children == null ? children : '';
+
+	const html = `<${name}${attrHTML}>${children}</${name}>`;
+
+	return html;
+}
+
+function getHTMLElementName(constructor: typeof HTMLElement) {
+	const definedName = (customElements as CustomElementRegistry & { getName(_constructor: typeof HTMLElement): string }).getName(constructor);
+	if (definedName) return definedName
+
+	const assignedName = constructor.name.replace(/^HTML|Element$/g, '').replace(/[A-Z]/g, '-$&').toLowerCase().replace(/^-/, 'html-')
+	return assignedName
+}
+
 function renderElement(name: string, { props: _props, children = '' }: SSRElement) {
 	// Do not print `hoist`, `lang`, `global`
 	const { lang: _, 'data-astro-id': astroId, 'define:vars': defineVars, ...props } = _props;