diff --git a/packages/ckeditor5-package-generator/lib/templates/sample/index.html b/packages/ckeditor5-package-generator/lib/templates/sample/index.html
index afca4361..ce6d2f63 100644
--- a/packages/ckeditor5-package-generator/lib/templates/sample/index.html
+++ b/packages/ckeditor5-package-generator/lib/templates/sample/index.html
@@ -94,6 +94,6 @@ <h3>Reporting issues</h3>
     </ul>
 </div>
 
-<script src="./script.dist.js"></script>
+<script src="./ckeditor.dist.js"></script>
 </body>
 </html>
diff --git a/packages/ckeditor5-package-tools/lib/utils/get-karma-config.js b/packages/ckeditor5-package-tools/lib/utils/get-karma-config.js
index f6d1ddb9..35f2483e 100644
--- a/packages/ckeditor5-package-tools/lib/utils/get-karma-config.js
+++ b/packages/ckeditor5-package-tools/lib/utils/get-karma-config.js
@@ -9,6 +9,7 @@
 
 const path = require( 'path' );
 const { getPostCssConfig } = require( '@ckeditor/ckeditor5-dev-utils' ).styles;
+const getThemePath = require( './get-theme-path' );
 
 /**
  * @param {Object} options
@@ -20,7 +21,7 @@ module.exports = options => {
 	const karmaConfig = {
 		basePath: options.cwd,
 
-		frameworks: [ 'mocha', 'sinon-chai' ],
+		frameworks: [ 'mocha', 'sinon-chai', 'webpack' ],
 
 		files: [ options.entryFile ],
 
@@ -133,7 +134,7 @@ function getWebpackConfiguration( options ) {
 			rules: [
 				{
 					test: /\.svg$/,
-					use: [ 'raw-loader' ]
+					use: 'raw-loader'
 				},
 				{
 					test: /\.css$/,
@@ -150,20 +151,20 @@ function getWebpackConfiguration( options ) {
 						'css-loader',
 						{
 							loader: 'postcss-loader',
-							options: getPostCssConfig( {
-								themeImporter: {
-									themePath: require.resolve(
-										path.join( options.cwd, 'node_modules', '@ckeditor', 'ckeditor5-theme-lark' )
-									)
-								},
-								minify: true
-							} )
+							options: {
+								postcssOptions: getPostCssConfig( {
+									themeImporter: {
+										themePath: getThemePath( options.cwd )
+									},
+									minify: true
+								} )
+							}
 						}
 					]
 				},
 				{
 					test: /\.(txt|html|rtf)$/,
-					use: [ 'raw-loader' ]
+					use: 'raw-loader'
 				}
 			]
 		},
@@ -176,7 +177,8 @@ function getWebpackConfiguration( options ) {
 	};
 
 	if ( options.sourceMap ) {
-		config.devtool = 'inline-source-map';
+		// Available list: https://webpack.js.org/configuration/devtool/.
+		config.devtool = 'eval-cheap-module-source-map';
 	}
 
 	if ( options.coverage ) {
@@ -185,7 +187,7 @@ function getWebpackConfiguration( options ) {
 				test: /\.js$/,
 				loader: 'istanbul-instrumenter-loader',
 				include: path.join( options.cwd, 'src' ),
-				query: {
+				options: {
 					esModules: true
 				}
 			}
diff --git a/packages/ckeditor5-package-tools/lib/utils/get-theme-path.js b/packages/ckeditor5-package-tools/lib/utils/get-theme-path.js
new file mode 100644
index 00000000..cea23fd0
--- /dev/null
+++ b/packages/ckeditor5-package-tools/lib/utils/get-theme-path.js
@@ -0,0 +1,26 @@
+/**
+ * @license Copyright (c) 2020-2021, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md.
+ */
+
+'use strict';
+
+/* eslint-env node */
+
+const path = require( 'path' );
+
+/**
+ * Returns an absolute path to the main file of the `@ckeditor/ckeditor5-theme-lark` package.
+ *
+ * The function does the same as what does `require.resolve()`. However, there is no option for mocking it in tests,
+ * hence the value is obtained manually.
+ *
+ * @param {String} cwd
+ * @return {String}
+ */
+module.exports = function getThemePath( cwd ) {
+	const packagePath = path.join( cwd, 'node_modules', '@ckeditor', 'ckeditor5-theme-lark' );
+	const packageJson = require( path.join( packagePath, 'package.json' ) );
+
+	return path.join( packagePath, packageJson.main );
+};
diff --git a/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-dll.js b/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-dll.js
index 17939372..96af7911 100644
--- a/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-dll.js
+++ b/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-dll.js
@@ -14,6 +14,7 @@ const webpack = require( 'webpack' );
 const TerserPlugin = require( 'terser-webpack-plugin' );
 const CKEditorWebpackPlugin = require( '@ckeditor/ckeditor5-dev-webpack-plugin' );
 const { getPostCssConfig } = require( '@ckeditor/ckeditor5-dev-utils' ).styles;
+const getThemePath = require( './get-theme-path' );
 
 module.exports = options => {
 	const packageJson = require( path.join( options.cwd, 'package.json' ) );
@@ -35,6 +36,10 @@ module.exports = options => {
 			manifest: require( ckeditor5manifestPath ),
 			scope: 'ckeditor5/src',
 			name: 'CKEditor5.dll'
+		} ),
+		new webpack.ProvidePlugin( {
+			process: 'process/browser',
+			Buffer: [ 'buffer', 'Buffer' ]
 		} )
 	];
 
@@ -102,12 +107,14 @@ module.exports = options => {
 						'css-loader',
 						{
 							loader: 'postcss-loader',
-							options: getPostCssConfig( {
-								themeImporter: {
-									themePath: getThemePath( options.cwd )
-								},
-								minify: true
-							} )
+							options: {
+								postcssOptions: getPostCssConfig( {
+									themeImporter: {
+										themePath: getThemePath( options.cwd )
+									},
+									minify: true
+								} )
+							}
 						}
 					]
 				}
@@ -115,19 +122,3 @@ module.exports = options => {
 		}
 	};
 };
-
-/**
- * Returns an absolute path to the main file of the `@ckeditor/ckeditor5-theme-lark` package.
- *
- * The function does the same as what does `require.resolve()`. However, there is no option for mocking it in tests,
- * hence the value is obtained manually.
- *
- * @param {String} cwd
- * @return {String}
- */
-function getThemePath( cwd ) {
-	const packagePath = path.join( cwd, 'node_modules', '@ckeditor', 'ckeditor5-theme-lark' );
-	const packageJson = require( path.join( packagePath, 'package.json' ) );
-
-	return path.join( packagePath, packageJson.main );
-}
diff --git a/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-server.js b/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-server.js
index 2921b6c4..7901ab3d 100644
--- a/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-server.js
+++ b/packages/ckeditor5-package-tools/lib/utils/get-webpack-config-server.js
@@ -11,11 +11,16 @@ const path = require( 'path' );
 const webpack = require( 'webpack' );
 const CKEditorWebpackPlugin = require( '@ckeditor/ckeditor5-dev-webpack-plugin' );
 const { getPostCssConfig } = require( '@ckeditor/ckeditor5-dev-utils' ).styles;
+const getThemePath = require( './get-theme-path' );
 
 module.exports = options => {
 	const webpackPlugins = [
 		new webpack.DefinePlugin( {
 			EDITOR_LANGUAGE: JSON.stringify( options.language )
+		} ),
+		new webpack.ProvidePlugin( {
+			process: 'process/browser',
+			Buffer: [ 'buffer', 'Buffer' ]
 		} )
 	];
 
@@ -38,7 +43,7 @@ module.exports = options => {
 		entry: path.join( options.cwd, 'sample', 'ckeditor.js' ),
 
 		output: {
-			filename: 'script.dist.js',
+			filename: 'ckeditor.dist.js',
 			path: path.join( options.cwd, 'sample' )
 		},
 
@@ -59,7 +64,7 @@ module.exports = options => {
 			rules: [
 				{
 					test: /\.svg$/,
-					use: [ 'raw-loader' ]
+					use: 'raw-loader'
 				},
 				{
 					test: /\.css$/,
@@ -76,14 +81,14 @@ module.exports = options => {
 						'css-loader',
 						{
 							loader: 'postcss-loader',
-							options: getPostCssConfig( {
-								themeImporter: {
-									themePath: require.resolve(
-										path.join( options.cwd, 'node_modules', '@ckeditor', 'ckeditor5-theme-lark' )
-									)
-								},
-								minify: true
-							} )
+							options: {
+								postcssOptions: getPostCssConfig( {
+									themeImporter: {
+										themePath: getThemePath( options.cwd )
+									},
+									minify: true
+								} )
+							}
 						}
 					]
 				}
diff --git a/packages/ckeditor5-package-tools/package.json b/packages/ckeditor5-package-tools/package.json
index b70a3c8b..23a71335 100644
--- a/packages/ckeditor5-package-tools/package.json
+++ b/packages/ckeditor5-package-tools/package.json
@@ -14,6 +14,7 @@
     "@ckeditor/ckeditor5-dev-utils": "^26.0.0",
     "@ckeditor/ckeditor5-dev-env": "^26.0.0",
     "@ckeditor/ckeditor5-dev-webpack-plugin": "^26.0.0",
+    "buffer": "^6.0.3",
     "chai": "^4.3.4",
     "css-loader": "^5.2.7",
     "istanbul-instrumenter-loader": "^3.0.1",
@@ -26,19 +27,21 @@
     "karma-sinon": "^1.0.5",
     "karma-sinon-chai": "^2.0.2",
     "karma-sourcemap-loader": "^0.3.8",
-    "karma-webpack": "^4.0.2",
+    "karma-webpack": "^5.0.0",
     "minimist": "^1.2.5",
     "mocha": "^7.2.0",
-    "postcss-loader": "^3.0.0",
+    "process": "^0.11.10",
+    "postcss": "^7.0.39",
+    "postcss-loader": "^4.3.0",
     "raw-loader": "^4.0.2",
     "sinon": "^9.2.4",
     "sinon-chai": "^3.7.0",
-    "style-loader": "^1.3.0",
+    "style-loader": "^2.0.0",
     "stylelint": "^13.13.1",
     "stylelint-config-ckeditor5": "^2.0.1",
     "terser-webpack-plugin": "^3.0.2",
-    "webpack": "^4.46.0",
-    "webpack-dev-server": "^4.1.0"
+    "webpack": "^5.58.1",
+    "webpack-dev-server": "^4.3.1"
   },
   "devDependencies": {
     "chai": "^4.3.4",
diff --git a/packages/ckeditor5-package-tools/tests/utils/generate-entry-file.js b/packages/ckeditor5-package-tools/tests/utils/generate-entry-file.js
index 41dfdc4c..27524556 100644
--- a/packages/ckeditor5-package-tools/tests/utils/generate-entry-file.js
+++ b/packages/ckeditor5-package-tools/tests/utils/generate-entry-file.js
@@ -54,10 +54,9 @@ describe( 'lib/utils/generate-entry-file', () => {
 	} );
 
 	afterEach( () => {
-		mockery.deregisterAll();
-		mockery.disable();
 		sinon.restore();
 		clock.restore();
+		mockery.disable();
 	} );
 
 	it( 'should be a function', () => {
diff --git a/packages/ckeditor5-package-tools/tests/utils/get-karma-config.js b/packages/ckeditor5-package-tools/tests/utils/get-karma-config.js
index 047bb082..2847b51f 100644
--- a/packages/ckeditor5-package-tools/tests/utils/get-karma-config.js
+++ b/packages/ckeditor5-package-tools/tests/utils/get-karma-config.js
@@ -5,17 +5,286 @@
 
 'use strict';
 
+const sinon = require( 'sinon' );
+const mockery = require( 'mockery' );
 const expect = require( 'chai' ).expect;
 
 describe( 'lib/utils/get-karma-config', () => {
-	let getKarmaConfig;
+	let getKarmaConfig, stubs;
+
+	const cwd = '/process/cwd';
 
 	beforeEach( () => {
+		mockery.enable( {
+			useCleanCache: true,
+			warnOnReplace: false,
+			warnOnUnregistered: false
+		} );
+
+		stubs = {
+			packageJson: {
+				name: '@ckeditor/ckeditor5-foo'
+			},
+			path: {
+				join: sinon.stub().callsFake( ( ...chunks ) => chunks.join( '/' ) ),
+				resolve: sinon.stub().callsFake( file => `/process/cwd/${ file }` )
+			},
+			devUtils: {
+				styles: {
+					getPostCssConfig: sinon.stub()
+				}
+			},
+			getThemePath: sinon.stub()
+		};
+
+		mockery.registerMock( 'path', stubs.path );
+		mockery.registerMock( '@ckeditor/ckeditor5-dev-utils', stubs.devUtils );
+		mockery.registerMock( './get-theme-path', stubs.getThemePath );
+
 		getKarmaConfig = require( '../../lib/utils/get-karma-config' );
 	} );
 
+	afterEach( () => {
+		sinon.restore();
+		mockery.disable();
+	} );
+
 	it( 'should be a function', () => {
 		expect( getKarmaConfig ).to.be.a( 'function' );
 	} );
-	// TODO: Add tests.
+
+	it( 'defines "webpack" in the "#frameworks" property', () => {
+		const config = getKarmaConfig( { cwd } );
+
+		expect( config.frameworks ).to.be.an( 'array' );
+		expect( config.frameworks ).to.contain( 'webpack' );
+	} );
+
+	it( 'uses no sandbox version of Chrome when executing on CI', () => {
+		const envCI = process.env.CI;
+		process.env.CI = true;
+
+		const config = getKarmaConfig( { cwd } );
+
+		expect( config.browsers ).to.deep.equal( [ 'CHROME_TRAVIS_CI' ] );
+
+		process.env.CI = envCI;
+	} );
+
+	it( 'enables watching source files when passed the watch option', () => {
+		const config = getKarmaConfig( { cwd, verbose: true } );
+
+		expect( config.webpackMiddleware ).to.deep.equal( {
+			noInfo: false
+		} );
+	} );
+
+	it( 'enables printing webpack logs when passed the verbose option', () => {
+		const config = getKarmaConfig( { cwd, watch: true } );
+
+		expect( config.autoWatch ).to.equal( true );
+		expect( config.singleRun ).to.equal( false );
+	} );
+
+	describe( 'webpack configuration', () => {
+		describe( 'loaders', () => {
+			let webpackConfig;
+
+			beforeEach( () => {
+				stubs.getThemePath.returns( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css' );
+				stubs.devUtils.styles.getPostCssConfig.returns( { foo: true } );
+
+				webpackConfig = getKarmaConfig( { cwd } ).webpack;
+			} );
+
+			describe( '*.svg', () => {
+				let loader;
+
+				beforeEach( () => {
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'svg' ) );
+
+					expect( loader ).is.an( 'object' );
+				} );
+
+				it( 'uses "raw-loader" for providing files', () => {
+					expect( loader.use ).to.equal( 'raw-loader' );
+				} );
+
+				it( 'loads paths that end with the ".svg" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.svg' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.svg' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+				} );
+			} );
+
+			describe( '*.css', () => {
+				let loader;
+
+				beforeEach( () => {
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'css' ) );
+
+					expect( loader ).is.an( 'object' );
+					expect( loader.use ).is.an( 'array' );
+					expect( loader.use ).to.lengthOf( 3 );
+				} );
+
+				// Webpack processes loaders from the bottom, to the top. Hence, "postcss-loader" will be called as the first one.
+				it( 'uses "postcss-loader" for processing CKEditor 5 assets', () => {
+					expect( stubs.getThemePath.calledOnce ).to.equal( true );
+					expect( stubs.getThemePath.firstCall.args[ 0 ] ).to.equal( '/process/cwd' );
+
+					expect( stubs.devUtils.styles.getPostCssConfig.calledOnce ).to.equal( true );
+					expect( stubs.devUtils.styles.getPostCssConfig.firstCall.firstArg ).to.deep.equal( {
+						minify: true,
+						themeImporter: {
+							themePath: '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css'
+						}
+					} );
+
+					const postcssLoader = loader.use.slice( -1 ).pop();
+
+					expect( postcssLoader ).to.be.an( 'object' );
+
+					expect( postcssLoader ).to.haveOwnProperty( 'loader' );
+					expect( postcssLoader.loader ).to.equal( 'postcss-loader' );
+
+					expect( postcssLoader ).to.haveOwnProperty( 'options' );
+					expect( postcssLoader.options ).to.deep.equal( {
+						postcssOptions: { foo: true }
+					} );
+				} );
+
+				it( 'uses "css-loader" to resolve imports from JS files', () => {
+					const cssLoader = loader.use.slice( 1, -1 ).pop();
+
+					expect( cssLoader ).to.equal( 'css-loader' );
+				} );
+
+				it( 'uses "style-loader" for injecting processed styles on a page', () => {
+					const styleLoader = loader.use.slice( 0 ).shift();
+
+					expect( styleLoader ).to.haveOwnProperty( 'loader' );
+					expect( styleLoader.loader ).to.equal( 'style-loader' );
+
+					expect( styleLoader ).to.haveOwnProperty( 'options' );
+					expect( styleLoader.options ).to.deep.equal( {
+						injectType: 'singletonStyleTag',
+						attributes: {
+							'data-cke': true
+						}
+					} );
+				} );
+
+				it( 'loads paths that end with the ".css" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.html' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.html' ).to.not.match( loader.test );
+				} );
+			} );
+
+			describe( '*.txt', () => {
+				let loader;
+
+				beforeEach( () => {
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'txt' ) );
+
+					expect( loader ).is.an( 'object' );
+				} );
+
+				it( 'uses "raw-loader" for providing files', () => {
+					expect( loader.use ).to.equal( 'raw-loader' );
+				} );
+
+				it( 'loads paths that end with the ".txt" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/assets/ckeditor.txt' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\assets\\ckeditor.txt' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+				} );
+			} );
+
+			describe( '*.html', () => {
+				let loader;
+
+				beforeEach( () => {
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'html' ) );
+
+					expect( loader ).is.an( 'object' );
+				} );
+
+				it( 'uses "raw-loader" for providing files', () => {
+					expect( loader.use ).to.equal( 'raw-loader' );
+				} );
+
+				it( 'loads paths that end with the ".html" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/assets/ckeditor.html' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\assets\\ckeditor.html' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+				} );
+			} );
+
+			describe( '*.rtf', () => {
+				let loader;
+
+				beforeEach( () => {
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'rtf' ) );
+
+					expect( loader ).is.an( 'object' );
+				} );
+
+				it( 'uses "raw-loader" for providing files', () => {
+					expect( loader.use ).to.equal( 'raw-loader' );
+				} );
+
+				it( 'loads paths that end with the ".rtf" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/assets/ckeditor.rtf' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\assets\\ckeditor.rtf' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+				} );
+			} );
+
+			describe( '*.js (code coverage)', () => {
+				let loader;
+
+				beforeEach( () => {
+					webpackConfig = getKarmaConfig( { cwd, coverage: true } ).webpack;
+
+					loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'js' ) );
+
+					expect( loader ).is.an( 'object' );
+				} );
+
+				it( 'uses "istanbul-instrumenter-loader" for providing files', () => {
+					expect( loader.loader ).to.equal( 'istanbul-instrumenter-loader' );
+					expect( loader.include ).to.equal( '/process/cwd/src' );
+					expect( loader.options ).to.deep.equal( {
+						esModules: true
+					} );
+				} );
+
+				it( 'loads paths that end with the ".js" suffix', () => {
+					expect( '/Users/ckeditor/ckeditor5-foo/src/ckeditor.js' ).to.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\src\\ckeditor.js' ).to.match( loader.test );
+
+					expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+					expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+				} );
+			} );
+		} );
+
+		it( 'allows enabling source maps', () => {
+			const config = getKarmaConfig( { cwd, sourceMap: true } );
+
+			expect( config.webpack.devtool ).to.equal( 'eval-cheap-module-source-map' );
+		} );
+	} );
 } );
diff --git a/packages/ckeditor5-package-tools/tests/utils/get-theme-path.js b/packages/ckeditor5-package-tools/tests/utils/get-theme-path.js
new file mode 100644
index 00000000..64947390
--- /dev/null
+++ b/packages/ckeditor5-package-tools/tests/utils/get-theme-path.js
@@ -0,0 +1,52 @@
+/**
+ * @license Copyright (c) 2020-2021, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md.
+ */
+
+'use strict';
+
+const mockery = require( 'mockery' );
+const sinon = require( 'sinon' );
+const expect = require( 'chai' ).expect;
+
+describe( 'lib/utils/get-theme-path', () => {
+	let getThemePath, stubs;
+
+	const cwd = '/process/cwd';
+
+	beforeEach( () => {
+		mockery.enable( {
+			useCleanCache: true,
+			warnOnReplace: false,
+			warnOnUnregistered: false
+		} );
+
+		stubs = {
+			packageJson: {
+				name: '@ckeditor/ckeditor5-theme-lark',
+				main: './theme/theme.css'
+			},
+			path: {
+				join: sinon.stub().callsFake( ( ...chunks ) => chunks.join( '/' ).replace( '/./', '/' ) )
+			}
+		};
+
+		mockery.registerMock( 'path', stubs.path );
+		mockery.registerMock( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme-lark/package.json', stubs.packageJson );
+
+		getThemePath = require( '../../lib/utils/get-theme-path' );
+	} );
+
+	afterEach( () => {
+		sinon.restore();
+		mockery.disable();
+	} );
+
+	it( 'should be a function', () => {
+		expect( getThemePath ).to.be.a( 'function' );
+	} );
+
+	it( 'returns an absolute path to an entry file of the "@ckeditor/ckeditor5-theme-lark" package', () => {
+		expect( getThemePath( cwd ) ).to.equal( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme-lark/theme/theme.css' );
+	} );
+} );
diff --git a/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-dll.js b/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-dll.js
index 0c58c0f5..9d9f4542 100644
--- a/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-dll.js
+++ b/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-dll.js
@@ -29,31 +29,48 @@ describe( 'lib/utils/get-webpack-config-dll', () => {
 				name: 'CKEditor5.dll',
 				content: {}
 			},
+			fs: {
+				existsSync: sinon.stub()
+			},
 			path: {
-				join: sinon.stub().callsFake( ( ...chunks ) => chunks.join( '/' ) ),
-				resolve: sinon.stub().callsFake( file => `/process/cwd/${ file }` )
+				join: sinon.stub().callsFake( ( ...chunks ) => chunks.join( '/' ) )
+			},
+			devUtils: {
+				styles: {
+					getPostCssConfig: sinon.stub()
+				}
 			},
+			getThemePath: sinon.stub(),
 			webpack: sinon.stub(),
-			dllReferencePlugin: sinon.stub()
+			dllReferencePlugin: sinon.stub(),
+			providePlugin: sinon.stub(),
+			devWebpackPlugin: sinon.stub()
 		};
 
 		stubs.webpack.DllReferencePlugin = function( ...args ) {
 			return stubs.dllReferencePlugin( ...args );
 		};
+		stubs.webpack.ProvidePlugin = function( ...args ) {
+			return stubs.providePlugin( ...args );
+		};
+
+		stubs.fs.existsSync.returns( false );
 
+		mockery.registerMock( 'fs', stubs.fs );
 		mockery.registerMock( 'path', stubs.path );
 		mockery.registerMock( 'webpack', stubs.webpack );
+		mockery.registerMock( '@ckeditor/ckeditor5-dev-utils', stubs.devUtils );
+		mockery.registerMock( '@ckeditor/ckeditor5-dev-webpack-plugin', stubs.devWebpackPlugin );
+		mockery.registerMock( './get-theme-path', stubs.getThemePath );
+
 		mockery.registerMock( '/process/cwd/node_modules/ckeditor5/build/ckeditor5-dll.manifest.json', stubs.ckeditor5manifest );
-		mockery.registerMock( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme-lark/package.json', {
-			main: './theme/theme.css'
-		} );
 		mockery.registerMock( '/process/cwd/package.json', stubs.packageJson );
 
 		getWebpackConfigDll = require( '../../lib/utils/get-webpack-config-dll' );
 	} );
 
 	afterEach( () => {
-		mockery.deregisterAll();
+		sinon.restore();
 		mockery.disable();
 	} );
 
@@ -61,6 +78,46 @@ describe( 'lib/utils/get-webpack-config-dll', () => {
 		expect( getWebpackConfigDll ).to.be.a( 'function' );
 	} );
 
+	it( 'loads "process" polyfill for webpack 5', () => {
+		getWebpackConfigDll( { cwd } );
+
+		expect( stubs.providePlugin.calledOnce ).to.equal( true );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.be.an( 'object' );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.have.property( 'process', 'process/browser' );
+	} );
+
+	it( 'loads "Buffer" polyfill for webpack 5', () => {
+		getWebpackConfigDll( { cwd } );
+
+		expect( stubs.providePlugin.calledOnce ).to.equal( true );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.be.an( 'object' );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.have.property( 'Buffer' );
+		expect( stubs.providePlugin.firstCall.firstArg.Buffer ).to.deep.equal( [ 'buffer', 'Buffer' ] );
+	} );
+
+	it( 'allows watching source files', () => {
+		const config = getWebpackConfigDll( { cwd, watch: true } );
+
+		expect( config.watch ).to.equal( true );
+	} );
+
+	it( 'produces translation files based on "*.po" files', () => {
+		stubs.fs.existsSync.returns( true );
+
+		getWebpackConfigDll( { cwd } );
+
+		expect( stubs.fs.existsSync.calledOnce ).to.equal( true );
+		expect( stubs.fs.existsSync.firstCall.firstArg ).to.equal( '/process/cwd/lang/translations/en.po' );
+
+		expect( stubs.devWebpackPlugin.calledOnce ).to.equal( true );
+		expect( stubs.devWebpackPlugin.firstCall.firstArg ).to.deep.equal( {
+			additionalLanguages: 'all',
+			language: 'en',
+			skipPluralFormFunction: true,
+			sourceFilesPattern: /^src[/\\].+\.js$/
+		} );
+	} );
+
 	describe( 'verifying exposed DLL names', () => {
 		it( 'returns "foo" for a window key and a file name for the "@ckeditor/ckeditor5-foo" package', () => {
 			const webpackConfig = getWebpackConfigDll( { cwd } );
@@ -78,5 +135,104 @@ describe( 'lib/utils/get-webpack-config-dll', () => {
 			expect( webpackConfig.output.library ).to.deep.equal( [ 'CKEditor5', 'fooBar' ] );
 		} );
 	} );
-	// TODO: Add more tests.
+
+	describe( 'loaders', () => {
+		let webpackConfig;
+
+		beforeEach( () => {
+			stubs.getThemePath.returns( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css' );
+			stubs.devUtils.styles.getPostCssConfig.returns( { foo: true } );
+
+			webpackConfig = getWebpackConfigDll( { cwd } );
+		} );
+
+		describe( '*.svg', () => {
+			let loader;
+
+			beforeEach( () => {
+				loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'svg' ) );
+
+				expect( loader ).is.an( 'object' );
+			} );
+
+			it( 'uses "raw-loader" for providing files', () => {
+				expect( loader.use ).to.deep.equal( [ 'raw-loader' ] );
+			} );
+
+			it( 'loads paths that end with the ".svg" suffix', () => {
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.svg' ).to.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.svg' ).to.match( loader.test );
+
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+			} );
+		} );
+
+		describe( '*.css', () => {
+			let loader;
+
+			beforeEach( () => {
+				loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'css' ) );
+
+				expect( loader ).is.an( 'object' );
+				expect( loader.use ).is.an( 'array' );
+				expect( loader.use ).to.lengthOf( 3 );
+			} );
+
+			// Webpack processes loaders from the bottom, to the top. Hence, "postcss-loader" will be called as the first one.
+			it( 'uses "postcss-loader" for processing CKEditor 5 assets', () => {
+				expect( stubs.getThemePath.calledOnce ).to.equal( true );
+				expect( stubs.getThemePath.firstCall.args[ 0 ] ).to.equal( '/process/cwd' );
+
+				expect( stubs.devUtils.styles.getPostCssConfig.calledOnce ).to.equal( true );
+				expect( stubs.devUtils.styles.getPostCssConfig.firstCall.firstArg ).to.deep.equal( {
+					minify: true,
+					themeImporter: {
+						themePath: '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css'
+					}
+				} );
+
+				const postcssLoader = loader.use.slice( -1 ).pop();
+
+				expect( postcssLoader ).to.be.an( 'object' );
+
+				expect( postcssLoader ).to.haveOwnProperty( 'loader' );
+				expect( postcssLoader.loader ).to.equal( 'postcss-loader' );
+
+				expect( postcssLoader ).to.haveOwnProperty( 'options' );
+				expect( postcssLoader.options ).to.deep.equal( {
+					postcssOptions: { foo: true }
+				} );
+			} );
+
+			it( 'uses "css-loader" to resolve imports from JS files', () => {
+				const cssLoader = loader.use.slice( 1, -1 ).pop();
+
+				expect( cssLoader ).to.equal( 'css-loader' );
+			} );
+
+			it( 'uses "style-loader" for injecting processed styles on a page', () => {
+				const styleLoader = loader.use.slice( 0 ).shift();
+
+				expect( styleLoader ).to.haveOwnProperty( 'loader' );
+				expect( styleLoader.loader ).to.equal( 'style-loader' );
+
+				expect( styleLoader ).to.haveOwnProperty( 'options' );
+				expect( styleLoader.options ).to.deep.equal( {
+					injectType: 'singletonStyleTag',
+					attributes: {
+						'data-cke': true
+					}
+				} );
+			} );
+
+			it( 'loads paths that end with the ".css" suffix', () => {
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.match( loader.test );
+
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.html' ).to.not.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.html' ).to.not.match( loader.test );
+			} );
+		} );
+	} );
 } );
diff --git a/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-server.js b/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-server.js
index 2d944340..89cf3591 100644
--- a/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-server.js
+++ b/packages/ckeditor5-package-tools/tests/utils/get-webpack-config-server.js
@@ -5,17 +5,229 @@
 
 'use strict';
 
+const mockery = require( 'mockery' );
+const sinon = require( 'sinon' );
 const expect = require( 'chai' ).expect;
 
 describe( 'lib/utils/get-webpack-config-server', () => {
-	let getWebpackConfigServer;
+	let getWebpackConfigServer, stubs;
+
+	const cwd = '/process/cwd';
 
 	beforeEach( () => {
+		mockery.enable( {
+			useCleanCache: true,
+			warnOnReplace: false,
+			warnOnUnregistered: false
+		} );
+
+		stubs = {
+			packageJson: {
+				name: '@ckeditor/ckeditor5-foo'
+			},
+			path: {
+				join: sinon.stub().callsFake( ( ...chunks ) => chunks.join( '/' ) ),
+				resolve: sinon.stub().callsFake( file => `/process/cwd/${ file }` )
+			},
+			devUtils: {
+				styles: {
+					getPostCssConfig: sinon.stub()
+				}
+			},
+			getThemePath: sinon.stub(),
+			webpack: sinon.stub(),
+			providePlugin: sinon.stub(),
+			definePlugin: sinon.stub(),
+			devWebpackPlugin: sinon.stub()
+		};
+
+		stubs.webpack.ProvidePlugin = function( ...args ) {
+			return stubs.providePlugin( ...args );
+		};
+
+		stubs.webpack.DefinePlugin = function( ...args ) {
+			return stubs.definePlugin( ...args );
+		};
+
+		mockery.registerMock( 'path', stubs.path );
+		mockery.registerMock( 'webpack', stubs.webpack );
+		mockery.registerMock( '@ckeditor/ckeditor5-dev-utils', stubs.devUtils );
+		mockery.registerMock( '@ckeditor/ckeditor5-dev-webpack-plugin', stubs.devWebpackPlugin );
+		mockery.registerMock( './get-theme-path', stubs.getThemePath );
+
 		getWebpackConfigServer = require( '../../lib/utils/get-webpack-config-server' );
 	} );
 
+	afterEach( () => {
+		sinon.restore();
+		mockery.disable();
+	} );
+
 	it( 'should be a function', () => {
 		expect( getWebpackConfigServer ).to.be.a( 'function' );
 	} );
-	// TODO: Add tests.
+
+	it( 'processes the "ckeditor.js" file', () => {
+		const config = getWebpackConfigServer( { cwd } );
+
+		expect( config.entry ).to.equal( '/process/cwd/sample/ckeditor.js' );
+		expect( config.output ).to.deep.equal( {
+			filename: 'ckeditor.dist.js',
+			path: '/process/cwd/sample'
+		} );
+	} );
+
+	it( 'loads "process" polyfill for webpack 5', () => {
+		getWebpackConfigServer( { cwd } );
+
+		expect( stubs.providePlugin.calledOnce ).to.equal( true );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.be.an( 'object' );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.have.property( 'process', 'process/browser' );
+	} );
+
+	it( 'loads "Buffer" polyfill for webpack 5', () => {
+		getWebpackConfigServer( { cwd } );
+
+		expect( stubs.providePlugin.calledOnce ).to.equal( true );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.be.an( 'object' );
+		expect( stubs.providePlugin.firstCall.firstArg ).to.have.property( 'Buffer' );
+		expect( stubs.providePlugin.firstCall.firstArg.Buffer ).to.deep.equal( [ 'buffer', 'Buffer' ] );
+	} );
+
+	it( 'defines the configuration for an HTTP server', () => {
+		const config = getWebpackConfigServer( { cwd } );
+
+		expect( config.devServer ).to.deep.equal( {
+			static: {
+				directory: '/process/cwd/sample'
+			},
+			compress: true
+		} );
+	} );
+
+	describe( 'sample language', () => {
+		it( 'passes the specified language directly to source file', () => {
+			getWebpackConfigServer( { cwd, language: 'en' } );
+
+			expect( stubs.definePlugin.calledOnce ).to.equal( true );
+			expect( stubs.definePlugin.firstCall.firstArg ).to.deep.equal( {
+				EDITOR_LANGUAGE: '"en"'
+			} );
+		} );
+
+		it( 'enables producing translations for non-English editors', () => {
+			getWebpackConfigServer( { cwd, language: 'pl' } );
+
+			expect( stubs.definePlugin.calledOnce ).to.equal( true );
+			expect( stubs.definePlugin.firstCall.firstArg ).to.deep.equal( {
+				EDITOR_LANGUAGE: '"pl"'
+			} );
+
+			expect( stubs.devWebpackPlugin.calledOnce ).to.equal( true );
+			expect( stubs.devWebpackPlugin.firstCall.firstArg ).to.deep.equal( {
+				language: 'pl',
+				sourceFilesPattern: /src[/\\].+\.js$/
+			} );
+		} );
+	} );
+
+	describe( 'loaders', () => {
+		let webpackConfig;
+
+		beforeEach( () => {
+			stubs.getThemePath.returns( '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css' );
+			stubs.devUtils.styles.getPostCssConfig.returns( { foo: true } );
+
+			webpackConfig = getWebpackConfigServer( { cwd } );
+		} );
+
+		describe( '*.svg', () => {
+			let loader;
+
+			beforeEach( () => {
+				loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'svg' ) );
+
+				expect( loader ).is.an( 'object' );
+			} );
+
+			it( 'uses "raw-loader" for providing files', () => {
+				expect( loader.use ).to.equal( 'raw-loader' );
+			} );
+
+			it( 'loads paths that end with the ".svg" suffix', () => {
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.svg' ).to.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.svg' ).to.match( loader.test );
+
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.not.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.not.match( loader.test );
+			} );
+		} );
+
+		describe( '*.css', () => {
+			let loader;
+
+			beforeEach( () => {
+				loader = webpackConfig.module.rules.find( loader => loader.test.toString().includes( 'css' ) );
+
+				expect( loader ).is.an( 'object' );
+				expect( loader.use ).is.an( 'array' );
+				expect( loader.use ).to.lengthOf( 3 );
+			} );
+
+			// Webpack processes loaders from the bottom, to the top. Hence, "postcss-loader" will be called as the first one.
+			it( 'uses "postcss-loader" for processing CKEditor 5 assets', () => {
+				expect( stubs.getThemePath.calledOnce ).to.equal( true );
+				expect( stubs.getThemePath.firstCall.args[ 0 ] ).to.equal( '/process/cwd' );
+
+				expect( stubs.devUtils.styles.getPostCssConfig.calledOnce ).to.equal( true );
+				expect( stubs.devUtils.styles.getPostCssConfig.firstCall.firstArg ).to.deep.equal( {
+					minify: true,
+					themeImporter: {
+						themePath: '/process/cwd/node_modules/@ckeditor/ckeditor5-theme/theme/theme.css'
+					}
+				} );
+
+				const postcssLoader = loader.use.slice( -1 ).pop();
+
+				expect( postcssLoader ).to.be.an( 'object' );
+
+				expect( postcssLoader ).to.haveOwnProperty( 'loader' );
+				expect( postcssLoader.loader ).to.equal( 'postcss-loader' );
+
+				expect( postcssLoader ).to.haveOwnProperty( 'options' );
+				expect( postcssLoader.options ).to.deep.equal( {
+					postcssOptions: { foo: true }
+				} );
+			} );
+
+			it( 'uses "css-loader" to resolve imports from JS files', () => {
+				const cssLoader = loader.use.slice( 1, -1 ).pop();
+
+				expect( cssLoader ).to.equal( 'css-loader' );
+			} );
+
+			it( 'uses "style-loader" for injecting processed styles on a page', () => {
+				const styleLoader = loader.use.slice( 0 ).shift();
+
+				expect( styleLoader ).to.haveOwnProperty( 'loader' );
+				expect( styleLoader.loader ).to.equal( 'style-loader' );
+
+				expect( styleLoader ).to.haveOwnProperty( 'options' );
+				expect( styleLoader.options ).to.deep.equal( {
+					injectType: 'singletonStyleTag',
+					attributes: {
+						'data-cke': true
+					}
+				} );
+			} );
+
+			it( 'loads paths that end with the ".css" suffix', () => {
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.css' ).to.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.css' ).to.match( loader.test );
+
+				expect( '/Users/ckeditor/ckeditor5-foo/theme/icons/ckeditor.html' ).to.not.match( loader.test );
+				expect( 'C:\\Users\\ckeditor\\ckeditor5-foo\\theme\\icons\\ckeditor.html' ).to.not.match( loader.test );
+			} );
+		} );
+	} );
 } );
diff --git a/scripts/ci/travis-check.js b/scripts/ci/travis-check.js
index 57952724..f3cd0b6b 100644
--- a/scripts/ci/travis-check.js
+++ b/scripts/ci/travis-check.js
@@ -121,7 +121,7 @@ function startDevelopmentServer( cwd ) {
 		// Hence, we can assume that the server is live at this stage.
 		sampleServer.stdout.on( 'data', data => {
 			const content = data.toString().slice( 0, -1 );
-			const endMatch = /\+ \d+ hidden modules/.test( content );
+			const endMatch = /webpack \d+\.\d+\.\d+ compiled successfully in \d+ ms/.test( content );
 
 			if ( endMatch ) {
 				return resolve( {