CSS Isolation

CSS Isolation is an important part of building production-ready single-spa systems. This lesson explains what css isolation means, how it works, and how to apply it with practical examples you can reuse.

CSS Isolation Overview

CSS Isolation lets you structure single-spa work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep css isolation focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

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.

CSS Isolation Example

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

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

Single-SPA Cheatsheet

Core single-spa APIs related to css isolation.

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 CSS Isolation Works in Single-SPA

CSS Isolation 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 CSS Isolation

For reliable micro frontends, css isolation 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 css isolation snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up css isolation.
Leaving css isolation untested, so regressions slip into production.
Over-engineering css isolation before you actually need the extra flexibility.

Key Takeaways

CSS Isolation is a core part of working effectively with single-spa.
Start small and keep css isolation focused on a single responsibility.
Apply consistent patterns so css isolation scales across your project.
Test and document css isolation to keep it maintainable over time.

Pro Tip

When you get stuck on css isolation, reduce it to the smallest reproducible example first — most single-spa issues become obvious once the noise is gone.