`loader` option in the `application` builder does not work for *.css files.

Reproduce the issue from the minimal example using angular.json, main.ts, ng build, and ng serve with the .css loader setting. Start by tracing the application builder's handling of CSS imported from TypeScript, then verify that the generated CSS is loaded by the served application and that the behavior is consistent for both build and serve modes.

Command

build, serve

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

No response

Description

When trying to import a *.css file inside a *.ts with the following syntax:

import './my-styles.css';

The built css file is never loaded into the application.
I was pointed out in https://github.com/angular/angular-cli/issues/27414 that I should use the new loader however that does not seem to work.

angular.json:

"loader": {
  ".css": "file"
}

ng serve output:

This would be useful in order to support third party libraries that use this kind of imports like monaco-editor.

Vite already handles this when we run ng serve --prebundle=true and it works when a library uses this imports, but it does not work with ng build or ng serve --prebundle=false.
Also esbuild is able to handle the import (https://esbuild.github.io/content-types/#css-from-js) and generate a css bundle for all the css imports that it finds in a entry point, for example I can see it generating a main.css file:

However, the css bundle outputted to the ./dist/** folder it is never loaded onto the app.

Minimal Reproduction

ng new my-app
cd ./my-app

Create a *.css file called "import-styles.css"
Import the file above into the main.ts or other *.ts file of the app.

ng serve

Exception or Error

No response

Your Environment

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Angular CLI: 17.3.3
Node: 20.11.1
Package Manager: npm 10.2.4
OS: win32 x64

Angular: 17.3.3
... animations, cli, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1703.3
@angular-devkit/build-angular   17.3.3
@angular-devkit/core            17.3.3
@angular-devkit/schematics      17.3.3
@schematics/angular             17.3.3
rxjs                            7.8.1
typescript                      5.4.4
zone.js                         0.14.4

Anything else relevant?

No response