Document noUncheckedSideEffectImports fixes for CSS packages

packages/tsconfig-reference/copy/en/options/noUncheckedSideEffectImports.md

@@ -33,8 +33,8 @@ When `--noUncheckedSideEffectImports` is enabled, TypeScript will error if it ca
 ```ts
 import "oops-this-module-does-not-exist";
 //     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-// error: Cannot find module 'oops-this-module-does-not-exist' or its corresponding
-//        type declarations.
+// error: Cannot find module or type declarations for side-effect import of
+//        'oops-this-module-does-not-exist'.
 ```
 
 When enabling this option, some working code may now receive an error, like in the CSS example above.
@@ -50,3 +50,57 @@ declare module "*.css" {}
 
 In fact, you might already have a file like this in your project!
 For example, running something like `vite init` might create a similar `vite-env.d.ts`.
+
+#### Side effect imports of CSS from packages
+
+A wildcard declaration like `declare module "*.css" {}` only matches specifiers that end in `.css`.
+If a package exposes its stylesheet through a subpath without that extension, and TypeScript can't find a declaration file for that subpath, importing it still causes an error:
+
+```ts
+import "library-name/css";
+//     ~~~~~~~~~~~~~~~~~~
+// error: Cannot find module or type declarations for side-effect import of
+//        'library-name/css'.
+```
+
+If you maintain such a package, the recommended fix is to ship a declaration file next to the stylesheet.
+For a stylesheet at `./dist/style.css`, that file is `./dist/style.d.css.ts`:
+
+```ts
+// ./dist/style.d.css.ts
+export {};
+```
+
+Users of the package then need to enable [`allowArbitraryExtensions`](#allowArbitraryExtensions).
+Otherwise, TypeScript reports error TS6263, because the import resolves to a declaration file for a file with a non-standard extension.
+
+Alternatively, you can add a `"types"` condition to the subpath in your `package.json`, pointing to a declaration file which only contains `export {};`.
+This works without any setting in your users' projects:
+
+```json
+{
+  "exports": {
+    "./css": {
+      "types": "./dist/style.d.ts",
+      "default": "./dist/style.css"
+    }
+  }
+}
+```
+
+```ts
+// ./dist/style.d.ts
+export {};
+```
+
+You can also expose the stylesheet through a subpath that ends in `.css`, like `library-name/style.css`, so that a `"*.css"` declaration in your users' projects matches it.
+
+If you use a package which does none of these, you can declare the specifier yourself in a global file:
+
+```ts
+// ./src/globals.d.ts
+declare module "library-name/css";
+```
+
+If a bundler already validates imports of non-code files like CSS in your project, you can instead set `noUncheckedSideEffectImports` to `false` and let the bundler handle them.
+TypeScript then no longer reports any unresolved side effect import.