In a runtime-integrated microfrontend system using Webpack Module Federation, concretely describe what happens in the browser from the moment a host page loads to the moment a remote microfrontend's UI is rendered.

## The two sides of the mechanism Module Federation is a Webpack, and now Rspack and Vite-plugin, capability that lets independently built and independently deployed JavaScript bundles share code with each other at runtime rather than being combined into one bundle at compile time. The mechanism has two sides. - **On the remote side**, a microfrontend, say checkout, is built with a federation config that declares an exposes map, such as exposing `./App` from `./src/App.tsx`, and emits a small, separately-hostable JavaScript file, conventionally named `remoteEntry.js`, alongside its normal application bundle. `remoteEntry.js` is not the application code itself; it's a lightweight container/runtime that knows how to fetch and instantiate the actual exposed modules on demand, plus metadata about which shared libraries the remote needs and which versions it can tolerate. - **On the host side**, the shell application declares remotes — a name-to-URL mapping — and can then write ordinary-looking dynamic imports for the remote module. ## What the browser actually does Concretely, the browser sequence is: 1. The host's own bundle loads and executes first, just like any single-page app. 2. When the host's router or component tree reaches a point that needs the checkout remote, the federation runtime issues a network request for checkout's `remoteEntry.js`. 3. That file executes and registers itself in a shared federation registry in the browser's global scope. 4. The runtime then reconciles the shared scope: if both host and remote declare `react` as shared and singleton, the runtime picks one instance, usually the host's if version-compatible, so there's only one React in the page — critical, because two React copies mounting into the same tree causes broken context and hook errors. 5. Once shared deps are resolved, the runtime dynamically imports the actual checkout application chunk, a second network request separate from `remoteEntry.js`, executes it, and the host mounts the returned component into the DOM. All of this happens live in the user's browser after the host's own build has already shipped, which is exactly why it counts as runtime rather than build-time integration: the host's build artifact never contained checkout's code at all, only a URL pointing at where to find it later. ## Why the pattern exists The reason this pattern exists is deploy independence with a lighter type-safety trade-off than fully disjoint apps like iframes or separate page loads: teams get to ship on their own schedule while still sharing a single-page-app feel and shared runtime dependencies, avoiding the cost of shipping a framework like React multiple times per page. The trade-off is that all the safety Module Federation gives up compared to build-time integration — no compile-time type checking across the host/remote boundary, no bundler-level tree-shaking across the boundary — has to be managed operationally instead of caught by the compiler. ## Failure modes Failure modes are concrete and recur in production. 1. **Version skew on shared dependencies** is the most common: if checkout requires a React version as singleton that the host's range doesn't overlap, Module Federation either throws a runtime unsatisfied-version error or, worse, silently picks an incompatible version and produces obscure hook or context bugs that never show up in either team's own test suite, since each team only tests against their own dependency versions. 2. **A second failure mode is a remote being unreachable** — the CDN serving `remoteEntry.js` returns a 404 or times out — which the host must handle explicitly with error boundaries, retries, or fallback UI, since there's no compiler to catch a missing import; without that handling, a checkout outage silently blanks the whole checkout section of an otherwise-healthy host page. 3. **A third is contract drift**: because there's no shared type-checked interface, the checkout team can rename a prop the host relies on and nothing fails until a user hits that code path in production, which is why teams doing this seriously invest in contract tests or generated types published alongside `remoteEntry.js`. ## Where it shows up A well-known real-world adopter pattern is **Zalando's** frontend platform and, more publicly, **IKEA** and **DAZN**, who documented moving multiple independently-owned page sections onto Module Federation specifically to let dozens of frontend teams deploy on independent schedules without shipping a duplicate React runtime per microfrontend — the shared-scope negotiation described above is precisely the piece that makes that duplication avoidable.