Implements low-level custom element support (#587)

* Implements low-level custom element support

* Changes based on self review

* Adds a changeset

* Polyfills are added even when not hydrating

* Remove hydrationMethod option

Punting on this idea until it's really needed.

.changeset/early-terms-bow.md

@@ -0,0 +1,6 @@
+---
+'astro': patch
+'@astrojs/parser': patch
+---
+
+Adds low-level custom element support that renderers can use to enable server side rendering. This will be used in renderers such as a Lit renderer.

docs/core-concepts/ui-renderers.md

@@ -100,10 +100,12 @@ Additionally, this entrypoint can define a [Snowpack plugin](https://www.snowpac
 export default {
   name: '@astrojs/renderer-xxx', // the renderer name
   client: './client.js', // relative path to the client entrypoint
-  server: './server.js', // relative path to the server entrypoint
+  server: './server.js', // optional, relative path to the server entrypoint
   snowpackPlugin: '@snowpack/plugin-xxx', // optional, the name of a snowpack plugin to inject
   snowpackPluginOptions: { example: true }, // optional, any options to be forwarded to the snowpack plugin
   knownEntrypoint: ['framework'], // optional, entrypoint modules that will be used by compiled source
+  external: ['dep'] // optional, dependencies that should not be built by snowpack
+  polyfills: ['./shadow-dom-polyfill.js'] // optional, module scripts that should be loaded before client hydration.
 };
 ```
 

package.json

@@ -29,7 +29,8 @@
     "scripts",
     "www",
     "docs-www",
-    "packages/astro/test/fixtures/builtins/packages/*"
+    "packages/astro/test/fixtures/builtins/packages/*",
+    "packages/astro/test/fixtures/custom-elements/my-component-lib"
   ],
   "volta": {
     "node": "14.16.1",
@@ -37,7 +38,8 @@
     "yarn": "1.22.10"
   },
   "dependencies": {
-    "@changesets/cli": "^2.16.0"
+    "@changesets/cli": "^2.16.0",
+    "camel-case": "^4.1.2"
   },
   "devDependencies": {
     "@typescript-eslint/eslint-plugin": "^4.22.0",

packages/astro-parser/src/index.ts

@@ -1,2 +1,3 @@
 export * from './interfaces';
+export * from './parse/utils/features';
 export { default as parse } from './parse/index.js';

packages/astro-parser/src/interfaces.ts

@@ -106,6 +106,9 @@ export interface Ast {
   css: Style;
   module: Script;
   // instance: Script;
+  meta: {
+    features: number;
+  }
 }
 
 export interface Warning {

packages/astro-parser/src/parse/index.ts

@@ -29,6 +29,7 @@ export class Parser {
   js: Script[] = [];
   meta_tags = {};
   last_auto_closed_tag?: LastAutoClosedTag;
+  feature_flags: 0
 
   constructor(template: string, options: ParserOptions) {
     if (typeof template !== 'string') {
@@ -266,5 +267,8 @@ export default function parse(template: string, options: ParserOptions = {}): As
     css: parser.css[0],
     // instance: instance_scripts[0],
     module: astro_scripts[0],
+    meta: {
+      features: parser.feature_flags
+    }
   };
 }

packages/astro-parser/src/parse/state/tag.ts

@@ -8,6 +8,7 @@ import { Parser } from '../index.js';
 import { Directive, DirectiveType, TemplateNode, Text } from '../../interfaces.js';
 import fuzzymatch from '../../utils/fuzzymatch.js';
 import list from '../../utils/list.js';
+import { FEATURE_CUSTOM_ELEMENT } from '../utils/features.js';
 
 const valid_tag_name = /^\!?[a-zA-Z]{1,}:?[a-zA-Z0-9\-]*/;
 
@@ -43,6 +44,7 @@ const SELF = /^astro:self(?=[\s/>])/;
 const COMPONENT = /^astro:component(?=[\s/>])/;
 const SLOT = /^astro:fragment(?=[\s/>])/;
 const HEAD = /^head(?=[\s/>])/;
+const CUSTOM_ELEMENT = /-/;
 
 function parent_is_head(stack) {
   let i = stack.length;
@@ -54,6 +56,7 @@ function parent_is_head(stack) {
   return false;
 }
 
+
 export default function tag(parser: Parser) {
   const start = parser.index++;
 
@@ -77,6 +80,10 @@ export default function tag(parser: Parser) {
 
   const name = read_tag_name(parser);
 
+  if(CUSTOM_ELEMENT.test(name)) {
+    parser.feature_flags |= FEATURE_CUSTOM_ELEMENT;
+  }
+
   if (meta_tags.has(name)) {
     const slug = meta_tags.get(name).toLowerCase();
     if (is_closing_tag) {

packages/astro-parser/src/parse/utils/features.ts

@@ -0,0 +1,2 @@
+
+export const FEATURE_CUSTOM_ELEMENT = 1 << 0;

packages/astro/src/@types/astro.ts

@@ -65,6 +65,8 @@ export interface TransformResult {
   css?: string;
   /** If this page exports a collection, the JS to be executed as a string */
   createCollection?: string;
+  hasCustomElements: boolean;
+  customElementCandidates: Map<string, string>;
 }
 
 export interface CompileResult {
@@ -180,7 +182,7 @@ export interface ComponentInfo {
 
 export type Components = Map<string, ComponentInfo>;
 
-type AsyncRendererComponentFn<U> = (Component: any, props: any, children: string | undefined) => Promise<U>;
+type AsyncRendererComponentFn<U> = (Component: any, props: any, children: string | undefined, options?: any) => Promise<U>;
 
 export interface Renderer {
   check: AsyncRendererComponentFn<boolean>;

packages/astro/src/compiler/codegen/index.ts

@@ -7,7 +7,7 @@ import 'source-map-support/register.js';
 import eslexer from 'es-module-lexer';
 import esbuild from 'esbuild';
 import path from 'path';
-import { parse } from '@astrojs/parser';
+import { parse, FEATURE_CUSTOM_ELEMENT } from '@astrojs/parser';
 import { walk, asyncWalk } from 'estree-walker';
 import _babelGenerator from '@babel/generator';
 import babelParser from '@babel/parser';
@@ -17,12 +17,14 @@ import { error, warn, parseError } from '../../logger.js';
 import { fetchContent } from './content.js';
 import { isFetchContent } from './utils.js';
 import { yellow } from 'kleur/colors';
-import { isComponentTag, positionAt } from '../utils.js';
+import { isComponentTag, isCustomElementTag, positionAt } from '../utils.js';
 import { renderMarkdown } from '@astrojs/markdown-support';
+import { camelCase } from 'camel-case';
 import { transform } from '../transform/index.js';
 import { PRISM_IMPORT } from '../transform/prism.js';
 import { nodeBuiltinsSet } from '../../node_builtins.js';
 import { readFileSync } from 'fs';
+import { pathToFileURL } from 'url';
 
 const traverse: typeof babelTraverse.default = (babelTraverse.default as any).default;
 
@@ -142,6 +144,13 @@ function generateAttributes(attrs: Record<string, string>): string {
   return result + '}';
 }
 
+function getComponentUrl(astroConfig: AstroConfig, url: string, parentUrl: string | URL){
+  const componentExt = path.extname(url);
+  const ext = PlainExtensions.has(componentExt) ? '.js' : `${componentExt}.js`;
+  const outUrl = new URL(url, parentUrl);
+  return '/_astro/' + outUrl.href.replace(astroConfig.projectRoot.href, '').replace(/\.[^.]+$/, ext);
+}
+
 interface GetComponentWrapperOptions {
   filename: string;
   astroConfig: AstroConfig;
@@ -151,36 +160,43 @@ const PlainExtensions = new Set(['.js', '.jsx', '.ts', '.tsx']);
 /** Generate Astro-friendly component import */
 function getComponentWrapper(_name: string, { url, importSpecifier }: ComponentInfo, opts: GetComponentWrapperOptions) {
   const { astroConfig, filename } = opts;
-  const currFileUrl = new URL(`file://${filename}`);
   const [name, kind] = _name.split(':');
-  const getComponentUrl = () => {
-    const componentExt = path.extname(url);
-    const ext = PlainExtensions.has(componentExt) ? '.js' : `${componentExt}.js`;
-    const outUrl = new URL(url, currFileUrl);
-    return '/_astro/' + outUrl.href.replace(astroConfig.projectRoot.href, '').replace(/\.[^.]+$/, ext);
-  };
-  const getComponentExport = () => {
-    switch (importSpecifier.type) {
-      case 'ImportDefaultSpecifier':
-        return { value: 'default' };
-      case 'ImportSpecifier': {
-        if (importSpecifier.imported.type === 'Identifier') {
-          return { value: importSpecifier.imported.name };
-        }
-        return { value: importSpecifier.imported.value };
-      }
-      case 'ImportNamespaceSpecifier': {
-        const [_, value] = name.split('.');
-        return { value };
-      }
-    }
-  };
 
-  const importInfo = kind ? { componentUrl: getComponentUrl(), componentExport: getComponentExport() } : {};
-  return {
-    wrapper: `__astro_component(${name}, ${JSON.stringify({ hydrate: kind, displayName: _name, ...importInfo })})`,
-    wrapperImport: `import {__astro_component} from 'astro/dist/internal/__astro_component.js';`,
-  };
+  // Special flow for custom elements
+  if (isCustomElementTag(name)) {
+    return {
+      wrapper: `__astro_component(...__astro_element_registry.astroComponentArgs("${name}", ${JSON.stringify({ hydrate: kind, displayName: _name })}))`,
+      wrapperImports: [`import {AstroElementRegistry} from 'astro/dist/internal/element-registry.js';`,`import {__astro_component} from 'astro/dist/internal/__astro_component.js';`],
+    };
+
+  } else {
+    const getComponentExport = () => {
+      switch (importSpecifier.type) {
+        case 'ImportDefaultSpecifier':
+          return { value: 'default' };
+        case 'ImportSpecifier': {
+          if (importSpecifier.imported.type === 'Identifier') {
+            return { value: importSpecifier.imported.name };
+          }
+          return { value: importSpecifier.imported.value };
+        }
+        case 'ImportNamespaceSpecifier': {
+          const [_, value] = name.split('.');
+          return { value };
+        }
+      }
+    };
+
+    const importInfo = kind ? {
+      componentUrl: getComponentUrl(astroConfig, url, pathToFileURL(filename)),
+      componentExport: getComponentExport()
+    } : {};
+
+    return {
+      wrapper: `__astro_component(${name}, ${JSON.stringify({ hydrate: kind, displayName: _name, ...importInfo })})`,
+      wrapperImports: [`import {__astro_component} from 'astro/dist/internal/__astro_component.js';`],
+    };
+  }
 }
 
 /**
@@ -251,19 +267,22 @@ interface CompileResult {
 }
 
 interface CodegenState {
-  filename: string;
-  fileID: string;
   components: Components;
   css: string[];
+  filename: string;
+  fileID: string;
   markers: {
     insideMarkdown: boolean | Record<string, any>;
   };
   exportStatements: Set<string>;
   importStatements: Set<string>;
+  customElementCandidates: Map<string, string>;
 }
 
 /** Compile/prepare Astro frontmatter scripts */
-function compileModule(module: Script, state: CodegenState, compileOptions: CompileOptions): CompileResult {
+function compileModule(ast: Ast, module: Script, state: CodegenState, compileOptions: CompileOptions): CompileResult {
+  const { astroConfig } = compileOptions;
+  const { filename } = state;
   const componentImports: ImportDeclaration[] = [];
   const componentProps: VariableDeclarator[] = [];
   const componentExports: ExportNamedDeclaration[] = [];
@@ -373,7 +392,14 @@ function compileModule(module: Script, state: CodegenState, compileOptions: Comp
         });
       }
       const { start, end } = componentImport;
-      state.importStatements.add(module.content.slice(start || undefined, end || undefined));
+      if(ast.meta.features & FEATURE_CUSTOM_ELEMENT &&  componentImport.specifiers.length === 0) {
+        // Add possible custom element, but only if the AST says there are custom elements.
+        const moduleImportName = camelCase(importUrl+ 'Module');
+        state.importStatements.add(`import * as ${moduleImportName} from '${importUrl}';\n`);
+        state.customElementCandidates.set(moduleImportName, getComponentUrl(astroConfig, importUrl, pathToFileURL(filename)));
+      } else {
+        state.importStatements.add(module.content.slice(start || undefined, end || undefined));
+      }
     }
 
     // TODO: actually expose componentExports other than __layout and __content
@@ -385,7 +411,6 @@ function compileModule(module: Script, state: CodegenState, compileOptions: Comp
     if (componentProps.length > 0) {
       const shortname = path.posix.relative(compileOptions.astroConfig.projectRoot.pathname, state.filename);
       const props = componentProps.map((prop) => (prop.id as Identifier)?.name).filter((v) => v);
-      console.log();
       warn(
         compileOptions.logging,
         shortname,
@@ -627,7 +652,7 @@ async function compileHtml(enterNode: TemplateNode, state: CodegenState, compile
                 const [componentNamespace] = componentName.split('.');
                 componentInfo = components.get(componentNamespace);
               }
-              if (!componentInfo) {
+              if (!componentInfo && !isCustomElementTag(componentName)) {
                 throw new Error(`Unknown Component: ${componentName}`);
               }
               if (componentName === 'Markdown') {
@@ -643,9 +668,11 @@ async function compileHtml(enterNode: TemplateNode, state: CodegenState, compile
                 curr = 'markdown';
                 return;
               }
-              const { wrapper, wrapperImport } = getComponentWrapper(name, componentInfo, { astroConfig, filename });
-              if (wrapperImport) {
-                importStatements.add(wrapperImport);
+              const { wrapper, wrapperImports } = getComponentWrapper(name, componentInfo ?? ({} as any), { astroConfig, filename });
+              if (wrapperImports) {
+                for(let wrapperImport of wrapperImports) {
+                  importStatements.add(wrapperImport);
+                }
               }
               if (curr === 'markdown') {
                 await pushMarkdownToBuffer();
@@ -794,9 +821,10 @@ export async function codegen(ast: Ast, { compileOptions, filename, fileID }: Co
     },
     importStatements: new Set(),
     exportStatements: new Set(),
+    customElementCandidates: new Map()
   };
 
-  const { script, createCollection } = compileModule(ast.module, state, compileOptions);
+  const { script, createCollection } = compileModule(ast, ast.module, state, compileOptions);
 
   compileCss(ast.css, state);
 
@@ -809,5 +837,7 @@ export async function codegen(ast: Ast, { compileOptions, filename, fileID }: Co
     html,
     css: state.css.length ? state.css.join('\n\n') : undefined,
     createCollection,
+    hasCustomElements: Boolean(ast.meta.features & FEATURE_CUSTOM_ELEMENT),
+    customElementCandidates: state.customElementCandidates
   };
 }

packages/astro/src/compiler/index.ts

@@ -106,7 +106,7 @@ interface CompileComponentOptions {
   isPage?: boolean;
 }
 /** Compiles an Astro component */
-export async function compileComponent(source: string, { compileOptions, filename, projectRoot, isPage }: CompileComponentOptions): Promise<CompileResult> {
+export async function compileComponent(source: string, { compileOptions, filename, projectRoot }: CompileComponentOptions): Promise<CompileResult> {
   const result = await transformFromSource(source, { compileOptions, filename, projectRoot });
   const site = compileOptions.astroConfig.buildOptions.site || `http://localhost:${compileOptions.astroConfig.devOptions.port}`;
 
@@ -116,6 +116,11 @@ import fetch from 'node-fetch';
 
 // <script astro></script>
 ${result.imports.join('\n')}
+${result.hasCustomElements ? `
+const __astro_element_registry = new AstroElementRegistry({
+  candidates: new Map([${Array.from(result.customElementCandidates).map(([identifier, url]) => `[${identifier}, '${url}']`).join(', ')}])
+});
+`.trim() : ''}
 
 // \`__render()\`: Render the contents of the Astro module.
 import { h, Fragment } from 'astro/dist/internal/h.js';

packages/astro/src/compiler/utils.ts

@@ -1,6 +1,11 @@
+/** Is the given string a custom-element tag? */
+export function isCustomElementTag(tag: string) {
+  return /[-]/.test(tag);
+}
+
 /** Is the given string a valid component tag */
 export function isComponentTag(tag: string) {
-  return /^[A-Z]/.test(tag) || /^[a-z]+\./.test(tag);
+  return /^[A-Z]/.test(tag) || /^[a-z]+\./.test(tag) || isCustomElementTag(tag);
 }
 
 export interface Position {

packages/astro/src/config_manager.ts

@@ -9,10 +9,13 @@ type RendererSnowpackPlugin = string | [string, any] | undefined;
 
 interface RendererInstance {
   name: string;
+  options: any;
   snowpackPlugin: RendererSnowpackPlugin;
-  client: string;
+  client: string | null;
   server: string;
   knownEntrypoints: string[] | undefined;
+  external: string[] | undefined;
+  polyfills: string[];
 }
 
 const CONFIG_MODULE_BASE_NAME = '__astro_config.js';
@@ -65,15 +68,25 @@ export class ConfigManager {
 
     const rendererInstances = (
       await Promise.all(
-        rendererNames.map((rendererName) => {
+        rendererNames.map(async (rendererName) => {
+          let _options: any = null;
+          if (Array.isArray(rendererName)) {
+            _options = rendererName[1];
+            rendererName = rendererName[0];
+          }
+
           const entrypoint = pathToFileURL(resolveDependency(rendererName)).toString();
-          return import(entrypoint);
+          const r = await import(entrypoint);
+          return {
+            raw: r.default,
+            options: _options
+          };
         })
       )
-    ).map(({ default: raw }, i) => {
+    ).map(({ raw, options }, i) => {
       const { name = rendererNames[i], client, server, snowpackPlugin: snowpackPluginName, snowpackPluginOptions } = raw;
 
-      if (typeof client !== 'string') {
+      if (typeof client !== 'string' && client != null) {
         throw new Error(`Expected "client" from ${name} to be a relative path to the client-side renderer!`);
       }
 
@@ -92,12 +105,17 @@ export class ConfigManager {
         throw new Error(`Expected the snowpackPlugin from ${name} to be a "string" but encountered "${typeof snowpackPluginName}"!`);
       }
 
+      const polyfillsNormalized = (raw.polyfills || []).map((p: string) => p.startsWith('.') ? path.join(name, p) : p);
+
       return {
         name,
+        options,
         snowpackPlugin,
-        client: path.join(name, raw.client),
+        client: raw.client ? path.join(name, raw.client) : null,
         server: path.join(name, raw.server),
         knownEntrypoints: raw.knownEntrypoints,
+        external: raw.external,
+        polyfills: polyfillsNormalized
       };
     });
 
@@ -107,16 +125,24 @@ export class ConfigManager {
   async buildSource(contents: string): Promise<string> {
     const renderers = await this.buildRendererInstances();
     const rendererServerPackages = renderers.map(({ server }) => server);
-    const rendererClientPackages = await Promise.all(renderers.map(({ client }) => this.resolvePackageUrl(client)));
+    const rendererClientPackages = await Promise.all(renderers.filter(({client}) => client).map(({ client }) => this.resolvePackageUrl(client!)));
+    const rendererPolyfills = await Promise.all(renderers.map(({ polyfills }) => Promise.all(polyfills.map(src => this.resolvePackageUrl(src)))));
+
+
     const result = /* js */ `${rendererServerPackages.map((pkg, i) => `import __renderer_${i} from "${pkg}";`).join('\n')}
 
 import { setRenderers } from 'astro/dist/internal/__astro_component.js';
 
-let rendererSources = [${rendererClientPackages.map((pkg) => `"${pkg}"`).join(', ')}];
-let renderers = [${rendererServerPackages.map((_, i) => `__renderer_${i}`).join(', ')}];
+let rendererInstances = [${renderers.map((r, i) => `{
+  source: ${rendererClientPackages[i] ? `"${rendererClientPackages[i]}"` : 'null'},
+  renderer: __renderer_${i},
+  options: ${r.options ? JSON.stringify(r.options) : 'null'},
+  polyfills: ${JSON.stringify(rendererPolyfills[i])}
+}`).join(', ')}];
 
 ${contents}
 `;
+
     return result;
   }
 

packages/astro/src/frontend/__astro_config.ts

@@ -1,6 +1,6 @@
-declare function setRenderers(sources: string[], renderers: any[]): void;
+import type { RendererInstance } from '../internal/__astro_component';
 
-declare let rendererSources: string[];
-declare let renderers: any[];
+declare function setRenderers(instances: RendererInstance[]): void;
+declare let rendererInstances: RendererInstance[];
 
-setRenderers(rendererSources, renderers);
+setRenderers(rendererInstances);

packages/astro/src/internal/__astro_component.ts

@@ -3,38 +3,64 @@ import hash from 'shorthash';
 import { valueToEstree, Value } from 'estree-util-value-to-estree';
 import { generate } from 'astring';
 import * as astro from './renderer-astro';
+import * as astroHtml from './renderer-html';
 
 // A more robust version alternative to `JSON.stringify` that can handle most values
 // see https://github.com/remcohaszing/estree-util-value-to-estree#readme
 const serialize = (value: Value) => generate(valueToEstree(value));
 
-let rendererSources: string[] = [];
-let renderers: Renderer[] = [];
-
-export function setRenderers(_rendererSources: string[], _renderers: Renderer[]) {
-  rendererSources = [''].concat(_rendererSources);
-  renderers = [astro as Renderer].concat(_renderers);
+export interface RendererInstance {
+  source: string | null;
+  renderer: Renderer;
+  options: any;
+  polyfills: string[];
 }
 
-const rendererCache = new WeakMap();
+const astroRendererInstance: RendererInstance = {
+  source: '',
+  renderer: astro as Renderer,
+  options: null,
+  polyfills: []
+};
+
+const astroHtmlRendererInstance: RendererInstance = {
+  source: '',
+  renderer: astroHtml as Renderer,
+  options: null,
+  polyfills: []
+};
+
+let rendererInstances: RendererInstance[] = [];
+
+export function setRenderers(_rendererInstances: RendererInstance[]) {
+  rendererInstances = [astroRendererInstance].concat(_rendererInstances);
+}
+
+function isCustomElementTag(name: string | Function) {
+  return typeof name === 'string' && /-/.test(name);
+}
+
+const rendererCache = new Map<any, RendererInstance>();
 
 /** For a given component, resolve the renderer. Results are cached if this instance is encountered again */
-async function resolveRenderer(Component: any, props: any = {}, children?: string) {
+async function resolveRenderer(Component: any, props: any = {}, children?: string): Promise<RendererInstance | undefined> {
   if (rendererCache.has(Component)) {
-    return rendererCache.get(Component);
+    return rendererCache.get(Component)!;
   }
 
   const errors: Error[] = [];
-  for (const __renderer of renderers) {
+  for (const instance of rendererInstances) {
+    const { renderer, options } = instance;
+
     // Yes, we do want to `await` inside of this loop!
     // __renderer.check can't be run in parallel, it
     // returns the first match and skips any subsequent checks
     try {
-      const shouldUse: boolean = await __renderer.check(Component, props, children);
+      const shouldUse: boolean = await renderer.check(Component, props, children, options);
 
       if (shouldUse) {
-        rendererCache.set(Component, __renderer);
-        return __renderer;
+        rendererCache.set(Component, instance);
+        return instance;
       }
     } catch (err) {
       errors.push(err);
@@ -47,26 +73,39 @@ async function resolveRenderer(Component: any, props: any = {}, children?: strin
   }
 }
 
-interface AstroComponentProps {
+export interface AstroComponentProps {
   displayName: string;
   hydrate?: 'load' | 'idle' | 'visible';
   componentUrl?: string;
   componentExport?: { value: string; namespace?: boolean };
 }
 
-/** For hydrated components, generate a <script type="module"> to load the component */
-async function generateHydrateScript({ renderer, astroId, props }: any, { hydrate, componentUrl, componentExport }: Required<AstroComponentProps>) {
-  const rendererSource = rendererSources[renderers.findIndex((r) => r === renderer)];
+interface HydrateScriptOptions {
+  instance: RendererInstance;
+  astroId: string;
+  props: any;
+}
 
-  const script = `<script type="module">
+/** For hydrated components, generate a <script type="module"> to load the component */
+async function generateHydrateScript({ instance, astroId, props }: HydrateScriptOptions, { hydrate, componentUrl, componentExport }: Required<AstroComponentProps>) {
+  const { source } = instance;
+
+  const hydrationSource = source ? `
+  const [{ ${componentExport.value}: Component }, { default: hydrate }] = await Promise.all([import("${componentUrl}"), import("${source}")]);
+  return (el, children) => hydrate(el)(Component, ${serialize(props)}, children);
+`.trim() : `
+  await import("${componentUrl}");
+  return () => {};
+`.trim()
+
+  const hydrationScript = `<script type="module">
 import setup from '/_astro_frontend/hydrate/${hydrate}.js';
 setup("${astroId}", async () => {
-  const [{ ${componentExport.value}: Component }, { default: hydrate }] = await Promise.all([import("${componentUrl}"), import("${rendererSource}")]);
-  return (el, children) => hydrate(el)(Component, ${serialize(props)}, children);
+  ${hydrationSource}
 });
 </script>`;
 
-  return script;
+  return hydrationScript;
 }
 
 const getComponentName = (Component: any, componentProps: any) => {
@@ -85,25 +124,35 @@ const getComponentName = (Component: any, componentProps: any) => {
 export const __astro_component = (Component: any, componentProps: AstroComponentProps = {} as any) => {
   if (Component == null) {
     throw new Error(`Unable to render ${componentProps.displayName} because it is ${Component}!\nDid you forget to import the component or is it possible there is a typo?`);
-  } else if (typeof Component === 'string') {
+  } else if (typeof Component === 'string' && !isCustomElementTag(Component)) {
     throw new Error(`Astro is unable to render ${componentProps.displayName}!\nIs there a renderer to handle this type of component defined in your Astro config?`);
   }
 
   return async (props: any, ..._children: string[]) => {
     const children = _children.join('\n');
-    let renderer = await resolveRenderer(Component, props, children);
+    let instance = await resolveRenderer(Component, props, children);
 
-    if (!renderer) {
-      // If the user only specifies a single renderer, but the check failed
-      // for some reason... just default to their preferred renderer.
-      renderer = rendererSources.length === 2 ? renderers[1] : null;
+    if (!instance) {
+      if(isCustomElementTag(Component)) {
+        instance = astroHtmlRendererInstance;
+      } else {
+        // If the user only specifies a single renderer, but the check failed
+        // for some reason... just default to their preferred renderer.
+        instance = rendererInstances.length === 2 ? rendererInstances[1] : undefined;
+      }
 
-      if (!renderer) {
+      if (!instance) {
         const name = getComponentName(Component, componentProps);
         throw new Error(`No renderer found for ${name}! Did you forget to add a renderer to your Astro config?`);
       }
     }
-    const { html } = await renderer.renderToStaticMarkup(Component, props, children);
+    let { html } = await instance.renderer.renderToStaticMarkup(Component, props, children, instance.options);
+
+    if(instance.polyfills.length) {
+      let polyfillScripts = instance.polyfills.map(src => `<script type="module" src="${src}"></script>`).join('');
+      html = html + polyfillScripts;
+    }
+
     // If we're NOT hydrating this component, just return the HTML
     if (!componentProps.hydrate) {
       // It's safe to remove <astro-fragment>, static content doesn't need the wrapper
@@ -112,7 +161,7 @@ export const __astro_component = (Component: any, componentProps: AstroComponent
 
     // If we ARE hydrating this component, let's generate the hydration script
     const astroId = hash.unique(html);
-    const script = await generateHydrateScript({ renderer, astroId, props }, componentProps as Required<AstroComponentProps>);
+    const script = await generateHydrateScript({ instance, astroId, props }, componentProps as Required<AstroComponentProps>);
     const astroRoot = `<astro-root uid="${astroId}">${html}</astro-root>`;
     return [astroRoot, script].join('\n');
   };

packages/astro/src/internal/element-registry.ts

@@ -0,0 +1,48 @@
+import type { AstroComponentProps } from './__astro_component';
+
+type ModuleCandidates = Map<any, string>;
+
+interface RegistryOptions {
+  candidates: ModuleCandidates;
+}
+class AstroElementRegistry {
+  private candidates: ModuleCandidates;
+  private cache: Map<string, string> = new Map();
+
+  constructor(options: RegistryOptions) {
+    this.candidates = options.candidates;
+  }
+
+  find(tagName: string) {
+    for(let [module, importSpecifier] of this.candidates) {
+      if(module && typeof module.tagName === 'string') {
+        if(module.tagName === tagName) {
+          // Found!
+          return importSpecifier;
+        }
+      }
+    }
+  }
+
+  findCached(tagName: string) {
+    if(this.cache.has(tagName)) {
+      return this.cache.get(tagName)!;
+    }
+    let specifier = this.find(tagName);
+    if(specifier) {
+      this.cache.set(tagName, specifier);
+    }
+    return specifier;
+  }
+
+  astroComponentArgs(tagName: string, props: AstroComponentProps) {
+    const specifier = this.findCached(tagName);
+    const outProps: AstroComponentProps = {
+      ...props,
+      componentUrl: specifier || props.componentUrl
+    };
+    return [tagName, outProps];
+  }
+}
+
+export { AstroElementRegistry };

packages/astro/src/internal/renderer-html.ts

@@ -0,0 +1,12 @@
+import { h } from './h';
+
+async function renderToStaticMarkup(tag: string, props: Record<string, any>, children: string) {
+  const html = await h(tag, props, Promise.resolve(children));
+  return {
+    html
+  };
+}
+
+export {
+  renderToStaticMarkup
+};

packages/astro/src/runtime.ts

@@ -366,11 +366,20 @@ async function createSnowpack(astroConfig: AstroConfig, options: CreateSnowpackO
   const rendererInstances = await configManager.buildRendererInstances();
   const knownEntrypoints: string[] = ['astro/dist/internal/__astro_component.js'];
   for (const renderer of rendererInstances) {
-    knownEntrypoints.push(renderer.server, renderer.client);
+    knownEntrypoints.push(renderer.server);
+    if(renderer.client) {
+      knownEntrypoints.push(renderer.client);
+    }
     if (renderer.knownEntrypoints) {
       knownEntrypoints.push(...renderer.knownEntrypoints);
     }
   }
+  const external = snowpackExternals.concat([]);
+  for(const renderer of rendererInstances) {
+    if(renderer.external) {
+      external.push(...renderer.external);
+    }
+  }
   const rendererSnowpackPlugins = rendererInstances.filter((renderer) => renderer.snowpackPlugin).map((renderer) => renderer.snowpackPlugin) as string | [string, any];
 
   const snowpackConfig = await loadConfiguration({
@@ -406,7 +415,7 @@ async function createSnowpack(astroConfig: AstroConfig, options: CreateSnowpackO
     },
     packageOptions: {
       knownEntrypoints,
-      external: snowpackExternals,
+      external,
     },
     alias: {
       ...Object.fromEntries(nodeBuiltinsMap),

packages/astro/test/custom-elements.test.js

@@ -0,0 +1,73 @@
+import { suite } from 'uvu';
+import * as assert from 'uvu/assert';
+import { doc } from './test-utils.js';
+import { setup } from './helpers.js';
+
+const CustomElements = suite('Custom Elements');
+
+setup(CustomElements, './fixtures/custom-elements');
+
+CustomElements('Work as constructors', async ({ runtime }) => {
+  const result = await runtime.load('/ctr');
+  if (result.error) throw new Error(result.error);
+
+  const $ = doc(result.contents);
+  assert.equal($('my-element').length, 1, 'Element rendered');
+  assert.equal($('my-element template[shadowroot=open]').length, 1, 'shadow rendered');
+});
+
+CustomElements('Works with exported tagName', async ({ runtime }) => {
+  const result = await runtime.load('/');
+  if (result.error) throw new Error(result.error);
+
+  const $ = doc(result.contents);
+  assert.equal($('my-element').length, 1, 'Element rendered');
+  assert.equal($('my-element template[shadowroot=open]').length, 1, 'shadow rendered');
+});
+
+CustomElements('Hydration works with exported tagName', async ({ runtime }) => {
+  const result = await runtime.load('/load');
+  if (result.error) throw new Error(result.error);
+
+  const html = result.contents;
+  const $ = doc(html);
+
+  // SSR
+  assert.equal($('my-element').length, 1, 'Element rendered');
+  assert.equal($('my-element template[shadowroot=open]').length, 1, 'shadow rendered');
+
+  // Hydration
+  assert.ok(new RegExp('/_astro/src/components/my-element.js').test(html), 'Component URL is included');
+});
+
+CustomElements('Polyfills are added before the hydration script', async ({ runtime }) => {
+  const result = await runtime.load('/load');
+  if (result.error) throw new Error(result.error);
+
+  const html = result.contents;
+  const $ = doc(html);
+
+  assert.equal($('script[type=module]').length, 2);
+  assert.equal($('script[type=module]').attr('src'), '/_snowpack/link/packages/astro/test/fixtures/custom-elements/my-component-lib/polyfill.js');
+});
+
+CustomElements('Polyfills are added even if not hydrating', async ({ runtime }) => {
+  const result = await runtime.load('/');
+  if (result.error) throw new Error(result.error);
+
+  const html = result.contents;
+  const $ = doc(html);
+
+  assert.equal($('script[type=module]').length, 1);
+  assert.equal($('script[type=module]').attr('src'), '/_snowpack/link/packages/astro/test/fixtures/custom-elements/my-component-lib/polyfill.js');
+});
+
+CustomElements('Custom elements not claimed by renderer are rendered as regular HTML', async ({ runtime }) => {
+  const result = await runtime.load('/nossr');
+  if (result.error) throw new Error(result.error);
+
+  const $ = doc(result.contents);
+  assert.equal($('client-element').length, 1, 'Rendered the client-only element');
+});
+
+CustomElements.run();

packages/astro/test/fixtures/custom-elements/astro.config.mjs

@@ -0,0 +1,6 @@
+
+export default {
+  renderers: [
+    '@astrojs/test-custom-element-renderer'
+  ]
+}

packages/astro/test/fixtures/custom-elements/my-component-lib/index.js

@@ -0,0 +1,8 @@
+
+export default {
+  name: '@astrojs/test-custom-element-renderer',
+  server: './server',
+  polyfills: [
+    './polyfill.js'
+  ]
+};

packages/astro/test/fixtures/custom-elements/my-component-lib/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@astrojs/test-custom-element-renderer",
+  "main": "index.js",
+  "version": "0.0.1",
+  "type": "module"
+}

packages/astro/test/fixtures/custom-elements/my-component-lib/polyfill.js

@@ -0,0 +1 @@
+console.log('this is a polyfill');

packages/astro/test/fixtures/custom-elements/my-component-lib/server.js

@@ -0,0 +1,30 @@
+function getConstructor(Component) {
+  if(typeof Component === 'string') {
+    const tagName = Component;
+    Component = customElements.get(tagName);
+  }
+  return Component;
+}
+
+function check(component) {
+  const Component = getConstructor(component);
+  if(typeof Component === 'function' && globalThis.HTMLElement.isPrototypeOf(Component)) {
+    return true;
+  }
+  return false;
+}
+
+function renderToStaticMarkup(component) {
+  const Component = getConstructor(component);
+  const el = new Component();
+  el.connectedCallback();
+  const html = `<${el.localName}><template shadowroot="open">${el.shadowRoot.innerHTML}</template>${el.innerHTML}</${el.localName}>`
+  return {
+    html
+  };
+}
+
+export default {
+  check,
+  renderToStaticMarkup
+};

packages/astro/test/fixtures/custom-elements/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "@astrojs/test-custom-elements",
+  "version": "0.0.1",
+  "dependencies": {
+    "@astrojs/test-custom-element-renderer": "0.0.1"
+  }
+}

packages/astro/test/fixtures/custom-elements/snowpack.config.json

@@ -0,0 +1,3 @@
+{
+  "workspaceRoot": "../../../../../"
+}

packages/astro/test/fixtures/custom-elements/src/components/custom-elements.shim.js

@@ -0,0 +1,29 @@
+
+globalThis.customElements = {
+  _elements: new Map(),
+  define(name, ctr) {
+    ctr.tagName = name;
+    this._elements.set(name, ctr);
+  },
+  get(name) {
+    return this._elements.get(name);
+  }
+};
+
+globalThis.HTMLElement = class {
+  attachShadow() {
+    this.shadowRoot = new HTMLElement();
+  }
+
+  get localName() {
+    return this.constructor.tagName;
+  }
+
+  get innerHTML() {
+    return this._innerHTML;
+  }
+
+  set innerHTML(val) {
+    this._innerHTML = val;
+  }
+};

packages/astro/test/fixtures/custom-elements/src/components/my-element.js

@@ -0,0 +1,15 @@
+import './custom-elements.shim.js';
+
+export const tagName = 'my-element';
+
+class MyElement extends HTMLElement {
+  connectedCallback() {
+    this.attachShadow({ mode: 'open' });
+    this.shadowRoot.innerHTML = `<span id="custom">Hello from a custom element!</span>`;
+    this.innerHTML = `<div id="custom-light">Light dom!</div>`
+  }
+}
+
+customElements.define(tagName, MyElement);
+
+export default MyElement;

packages/astro/test/fixtures/custom-elements/src/pages/ctr.astro

@@ -0,0 +1,16 @@
+---
+import MyElement from '../components/my-element.js';
+
+const title = 'My App';
+---
+
+<html>
+<head>
+  <title>{title}</title>
+</head>
+<body>
+  <h1>{title}</h1>
+
+  <MyElement />
+</body>
+</html>

packages/astro/test/fixtures/custom-elements/src/pages/index.astro

@@ -0,0 +1,15 @@
+---
+const title = 'My App';
+import '../components/my-element.js';
+---
+
+<html>
+<head>
+  <title>{title}</title>
+</head>
+<body>
+  <h1>{title}</h1>
+
+  <my-element></my-element>
+</body>
+</html>

packages/astro/test/fixtures/custom-elements/src/pages/load.astro

@@ -0,0 +1,15 @@
+---
+const title = 'My App';
+import '../components/my-element.js';
+---
+
+<html>
+<head>
+  <title>{title}</title>
+</head>
+<body>
+  <h1>{title}</h1>
+
+  <my-element:load></my-element:load>
+</body>
+</html>

packages/astro/test/fixtures/custom-elements/src/pages/nossr.astro

@@ -0,0 +1,14 @@
+---
+const title = 'My App';
+---
+
+<html>
+<head>
+  <title>{title}</title>
+</head>
+<body>
+  <h1>{title}</h1>
+
+  <client-element></client-element>
+</body>
+</html>

packages/astro/test/helpers.js

@@ -23,7 +23,7 @@ const MAX_SHUTDOWN_TIME = 3000; // max time shutdown() may take
  * @param {SetupOptions} setupOptions
  */
 export function setup(Suite, fixturePath, { runtimeOptions = {} } = {}) {
-  let runtime;
+  let runtime, createRuntimeError;
   const timers = {};
 
   Suite.before(async (context) => {
@@ -36,7 +36,11 @@ export function setup(Suite, fixturePath, { runtimeOptions = {} } = {}) {
     runtime = await createRuntime(astroConfig, {
       logging: { level: 'error', dest: process.stderr },
       ...runtimeOptions,
-    });
+    }).catch(err => { createRuntimeError = err; });
+
+    if(createRuntimeError) {
+      setTimeout(() => { throw createRuntimeError });
+    }
 
     context.runtime = runtime;