Shadow DOM Styling

In this lesson you will learn shadow dom styling in single-spa, why it matters within styling, and how to use it correctly with clear, copy-ready examples.

Shadow DOM Styling Overview

Shadow DOM Styling is a building block you will reach for often in single-spa. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn shadow dom styling properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real single-spa projects.

import { registerApplication, start } from 'single-spa';

registerApplication({
  name: '@org/app',
  app: () => System.import('@org/app'),
  activeWhen: ['/app'],
});

start();

single-spa orchestrates multiple framework apps on one page through a root config.

Shadow DOM Styling Example

registerApplication({
  name: '@org/app',
  app: () => System.import('@org/app'),
  activeWhen: ['/app'],
});
start();

Start from a minimal Shadow DOM Styling example and grow it only as needed.
Keep configuration explicit so Shadow DOM Styling behaves the same in every environment.
Name things clearly so teammates understand your Shadow DOM Styling at a glance.
Add tests around Shadow DOM Styling early to lock in expected behaviour.

Single-SPA Cheatsheet

Core single-spa APIs related to shadow dom styling.

API	Example	Purpose
registerApplication	`registerApplication({ name, app, activeWhen })`	Register a micro frontend
activeWhen	`activeWhen: ['/checkout']`	Route ownership
start	`start()`	Begin routing
bootstrap	`export async function bootstrap()`	One-time setup
mount	`export async function mount(props)`	Render the app
unmount	`export async function unmount(props)`	Clean up the app
import map	`systemjs-importmap`	Locate app bundles

How Shadow DOM Styling Works in Single-SPA

Shadow DOM Styling is part of how single-spa lets multiple applications — even in different frameworks — coexist on one page. A root config registers each app and controls when it is active.

single-spa orchestrates multiple framework apps on one page through a root config.

A root config registers apps and calls start().
Each app exports bootstrap, mount, and unmount lifecycles.
activeWhen decides which routes each app owns.
Import maps resolve each app's bundle at runtime.

Practical Guidance for Shadow DOM Styling

For reliable micro frontends, shadow dom styling should isolate failures and keep shared state minimal. Let each team own its app end to end while agreeing on a few shared contracts.

Concern	Recommendation
Isolation	One app's crash should not break others
Shared state	Prefer shared utility modules over globals
Routing	Keep activeWhen rules explicit and non-overlapping
Deployment	Release via import-map updates per app

Common Mistakes

Copying shadow dom styling snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up shadow dom styling.
Leaving shadow dom styling untested, so regressions slip into production.
Over-engineering shadow dom styling before you actually need the extra flexibility.

Key Takeaways

Shadow DOM Styling is a core part of working effectively with single-spa.
Start small and keep shadow dom styling focused on a single responsibility.
Apply consistent patterns so shadow dom styling scales across your project.
Test and document shadow dom styling to keep it maintainable over time.

Pro Tip

Pair shadow dom styling with automated tests from day one. It is far cheaper to catch single-spa regressions in CI than in production.