feat: run on webpack-dev-middleware's client, with compatibility kept here

.changeset/reuse-dev-middleware-client.md

@@ -0,0 +1,5 @@
+---
+"webpack-dev-server": minor
+---
+
+The browser client, the error overlay and both transports now come from webpack-dev-middleware, so this package has no client code of its own. Every option keeps its name and meaning, `allowedHosts` and the origin checks still apply here, and this server's `{ type }` messages are still sent next to the middleware's, so React Refresh keeps working. `client/index.js`, `client/socket.js` and `client/clients/WebSocketClient.js` still resolve while the client's internal files are gone, and the console wording, the overlay and the progress indicator are now the middleware's. `client.webSocketTransport: "sse"` adds Server-Sent Events, a universal build now runs in Node without the client throwing, and `sendMessage()`, `getClientEntry()` and `getClientHotEntry()` still work, a subclass's overrides included, but are deprecated.

.gitignore

@@ -26,3 +26,4 @@ test/fixtures/reload-config/main.css
 test/fixtures/reload-config-2/main.css
 test/fixtures/worker-config-dev-server-false/public
 !/test/fixtures/static-config/public/node_modules
+test/fixtures/compatibility-config/changing.cjs

README.md

@@ -259,10 +259,13 @@ See [**the documentation**][docs-url] for more use cases and options.
 
 ## Browser Support
 
-While `webpack-dev-server` transpiles the client (browser) scripts to an ES5
-state, the project only officially supports the _last two versions of major
-browsers_. We simply don't have the resources to support every whacky
-browser out there.
+The browser runtime is [webpack-dev-middleware]'s, which ships as ES5 and uses
+no built-in newer than ES5 beyond `Promise`, the transport in use and what hot
+module replacement itself needs. As before, the project only officially
+supports the _last two versions of major browsers_. We simply don't have the
+resources to support every whacky browser out there.
+
+[webpack-dev-middleware]: https://github.com/webpack/webpack-dev-middleware#client-options
 
 If you find a bug with an obscure / old browser, we would actively welcome a
 Pull Request to resolve the bug.