diff --git a/.gitattributes b/.gitattributes
index 289761dafa2..2a4681cce94 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,5 +1,35 @@
 # Improve diff output for PHP files.
-*.php                                                    diff=php
+*.php diff=php
+
+/.github              export-ignore
+/bin                  export-ignore
+/tests                export-ignore
+/.browserslistrc      export-ignore
+/.devlib              export-ignore
+/.editorconfig        export-ignore
+/.eslintignore        export-ignore
+/.eslintrc            export-ignore
+/.gitattributes       export-ignore
+/.gitignore           export-ignore
+/.gitmodules          export-ignore
+/.npmrc               export-ignore
+/.nvmrc               export-ignore
+/.phpcs.xml.dist      export-ignore
+/.phpstorm.meta.php   export-ignore
+/.rtlcssrc            export-ignore
+/.stylelintignore     export-ignore
+/.stylelintrc.json    export-ignore
+/babel.config.js      export-ignore
+/codecov.yml          export-ignore
+/Gruntfile.js         export-ignore
+/package.json         export-ignore
+/package-lock.json    export-ignore
+/phpstan.neon.dist    export-ignore
+/phpunit.xml.dist     export-ignore
+/postcss.config.js    export-ignore
+/scoper.inc.php       export-ignore
+/sizereport.config.js export-ignore
+/webpack.config.js    export-ignore
 
 # Mark generated files so diffs are hidden by default.
 *.snap                                                   linguist-generated=true
diff --git a/.github/workflows/build-test-measure.yml b/.github/workflows/build-test-measure.yml
index 1ee847a32f5..b81030c5390 100644
--- a/.github/workflows/build-test-measure.yml
+++ b/.github/workflows/build-test-measure.yml
@@ -387,14 +387,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
 
-      - name: Setup PHP
-        uses: shivammathur/setup-php@v2
-        with:
-          php-version: ${{ matrix.php }}
-          extensions: curl, date, dom, gd, iconv, json, libxml, mysql, spl
-          coverage: ${{ matrix.coverage && 'pcov' || 'none' }}
-          ini-values: pcov.directory=.
-
       - name: Shutdown default MySQL service
         run: sudo service mysql stop
 
@@ -425,8 +417,31 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-composer-
 
+        # PHP-Scoper only works on PHP 7.3+ but is still a necessary tool to generate the scoped dependencies. To work
+        # around this, a compatible PHP version is temporarily installed and then used to install the dependencies.
+        # Once that is done, the PHP version is replaced with the one meant to be used and the vendor directory is
+        # removed so that the appropriate dependencies can then be later installed.
+
+      - name: Setup PHP 7.4 (to run PHP Scoper)
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+
+      - name: Install Composer dependencies (and scope necessary dependencies)
+        run: |
+          composer install --prefer-dist --no-suggest --no-progress --no-interaction
+          rm -rf vendor
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: curl, date, dom, gd, iconv, json, libxml, mysql, spl
+          coverage: ${{ matrix.coverage && 'pcov' || 'none' }}
+          ini-values: pcov.directory=.
+
       - name: Install Composer dependencies
-        run: composer install --prefer-dist --ignore-platform-reqs --no-progress --no-interaction
+        run: composer install --prefer-dist --ignore-platform-reqs --no-progress --no-interaction --no-scripts
 
       - name: Get npm cache directory
         id: npm-cache
@@ -670,7 +685,7 @@ jobs:
       git-sha-8: ${{ steps.retrieve-git-sha-8.outputs.sha8 }}
     strategy:
       matrix:
-        build: ['dev', 'prod']
+        build: ['dev', 'prod', 'composer']
 
     steps:
       - name: Check out source files
@@ -755,18 +770,16 @@ jobs:
     runs-on: ubuntu-latest
     needs:
       - build-zip
-    steps:
-      - name: Download dev build
-        uses: actions/download-artifact@v2
-        with:
-          name: amp-${{ needs.build-zip.outputs.branch-name }}-${{ needs.build-zip.outputs.git-sha-8 }}-dev
-          path: builds/dev
+    strategy:
+      matrix:
+        build: [ 'dev', 'prod', 'composer' ]
 
-      - name: Download prod build
+    steps:
+      - name: Download ${{ matrix.build }} build
         uses: actions/download-artifact@v2
         with:
-          name: amp-${{ needs.build-zip.outputs.branch-name }}-${{ needs.build-zip.outputs.git-sha-8 }}-prod
-          path: builds/prod
+          name: amp-${{ needs.build-zip.outputs.branch-name }}-${{ needs.build-zip.outputs.git-sha-8 }}-${{ matrix.build }}
+          path: builds/${{ matrix.build }}
 
       - name: Setup Google Cloud SDK
         uses: google-github-actions/setup-gcloud@master
@@ -774,11 +787,8 @@ jobs:
           project_id: ${{ secrets.GCS_PROJECT_ID }}
           service_account_key: ${{ secrets.GCS_APPLICATION_CREDENTIALS }}
 
-      - name: Upload dev build to bucket
-        run: gsutil cp -r builds/dev/amp.zip gs://ampwp_github_artifacts/${{ github.ref }}/dev/amp.zip
-
-      - name: Upload prod build to bucket
-        run: gsutil cp -r builds/prod/amp.zip gs://ampwp_github_artifacts/${{ github.ref }}/prod/amp.zip
+      - name: Upload ${{ matrix.build }} build to bucket
+        run: gsutil cp -r builds/${{ matrix.build }}/amp.zip gs://ampwp_github_artifacts/${{ github.ref }}/${{ matrix.build }}/amp.zip
 
 #-----------------------------------------------------------------------------------------------------------------------
 
@@ -807,7 +817,8 @@ jobs:
         run: |
           body="Plugin builds for ${{ github.event.pull_request.head.sha }} are ready :bellhop_bell:!
           - Download [development build](https://storage.googleapis.com/ampwp_github_artifacts/${{ github.ref }}/dev/amp.zip)
-          - Download [production build](https://storage.googleapis.com/ampwp_github_artifacts/${{ github.ref }}/prod/amp.zip)"
+          - Download [production build](https://storage.googleapis.com/ampwp_github_artifacts/${{ github.ref }}/prod/amp.zip)
+          - Download [composer build](https://storage.googleapis.com/ampwp_github_artifacts/${{ github.ref }}/composer/amp.zip)"
           body="${body//$'\n'/'%0A'}"
           echo "::set-output name=body::$body"
 
diff --git a/.gitignore b/.gitignore
index 1044d8cf962..a02048459e7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,7 @@ built
 /phpcs.xml
 /phpunit.xml
 /*.sql
+/third-party
 
 # Generated via bin/transform-readme.php
 /readme.txt
diff --git a/.phpcs.xml.dist b/.phpcs.xml.dist
index 8bad0b81e96..90417f09b61 100644
--- a/.phpcs.xml.dist
+++ b/.phpcs.xml.dist
@@ -131,6 +131,10 @@
 		</properties>
 	</rule>
 
+	<rule ref="Squiz.Commenting.FileComment.Missing">
+		<exclude-pattern>scoper.inc.php</exclude-pattern>
+	</rule>
+
 	<arg value="s"/>
 	<arg name="extensions" value="php"/>
 	<file>.</file>
@@ -146,4 +150,5 @@
 	<exclude-pattern type="relative">^build/*</exclude-pattern>
 	<exclude-pattern>includes/sanitizers/class-amp-allowed-tags-generated.php</exclude-pattern>
 	<exclude-pattern>assets/js/*.asset.php</exclude-pattern>
+	<exclude-pattern>third-party/*</exclude-pattern>
 </ruleset>
diff --git a/Gruntfile.js b/Gruntfile.js
index 32385a415c1..1d1e71855af 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -3,7 +3,7 @@
 module.exports = function( grunt ) {
 	'use strict';
 
-	// Root paths to include in the plugin build ZIP when running `npm run build:prod`.
+	// Root paths to include in the plugin build ZIP.
 	const productionIncludedRootFiles = [
 		'LICENSE',
 		'amp.php',
@@ -12,7 +12,6 @@ module.exports = function( grunt ) {
 		'includes',
 		'src',
 		'templates',
-		'vendor',
 	];
 
 	// These patterns paths will be excluded from among the above directory.
@@ -43,6 +42,17 @@ module.exports = function( grunt ) {
 		'vendor/ampproject/amp-toolbox/conceptual-diagram.svg',
 		'vendor/ampproject/amp-toolbox/phpstan.neon.dist',
 		'vendor/bin',
+		'third-party/composer.json',
+		'scoper.inc.php',
+	];
+
+	// These will be removed from the Composer build of the plugin prior to creating a ZIP.
+	// ⚠️ Warning: These paths are passed straight to rm command in the shell, without any escaping.
+	const productionComposerExcludedFilePatterns = [
+		'vendor',
+		'composer.lock',
+		'third-party/composer.json',
+		'scoper.inc.php',
 	];
 
 	grunt.initConfig( {
@@ -81,10 +91,12 @@ module.exports = function( grunt ) {
 			composer_install: {
 				command: [
 					'if [ ! -e build ]; then echo "Run grunt build first."; exit 1; fi',
+					'mkdir -p build/vendor/bin',
+					'cp vendor/bin/php-scoper build/vendor/bin/',
 					'cd build',
 					'composer install --no-dev -o',
-					'composer remove cweagans/composer-patches --update-no-dev -o',
-					'rm -rf ' + productionVendorExcludedFilePatterns.join( ' ' ),
+					'COMPOSER_DISCARD_CHANGES=true composer remove --no-interaction --no-scripts --update-no-dev -o cweagans/composer-patches sabberworm/php-css-parser',
+					'rm -rf ' + ( 'composer' === process.env.BUILD_TYPE ? productionComposerExcludedFilePatterns : productionVendorExcludedFilePatterns ).join( ' ' ),
 				].join( ' && ' ),
 			},
 			create_build_zip: {
@@ -155,6 +167,9 @@ module.exports = function( grunt ) {
 			paths.push( 'readme.txt' );
 
 			paths.push( 'composer.*' ); // Copy in order to be able to do run composer_install.
+			paths.push( 'scoper.inc.php' ); // Copy in order generate scoped Composer dependencies.
+
+			//  Also copy recently built assets.
 			paths.push( 'assets/js/**/*.js' );
 			paths.push( 'assets/js/**/*.asset.php' );
 			paths.push( 'assets/css/*.css' );
@@ -164,30 +179,44 @@ module.exports = function( grunt ) {
 				paths.push( 'assets/css/*.css.map' );
 			}
 
+			// Get build version from amp.php.
+			const versionRegex = /(\*\s+Version:\s+)(?<version>\d+(\.\d+)+)(?<identifier>-\w+)?/;
+			const { groups: matches } = grunt.file.read( 'amp.php' ).match( versionRegex );
+
+			if ( ! matches || ! matches.version ) {
+				throw new Error( 'Plugin version could not be retrieved from amp.php' );
+			}
+
+			const version = matches.version;
+
 			grunt.config.set( 'copy', {
 				build: {
 					src: paths,
 					dest: 'build',
 					expand: true,
 					options: {
-						noProcess: [ '*/**', 'LICENSE' ], // That is, only process amp.php and README.md.
+						noProcess: [ '**/*', '!amp.php', '!composer.json' ],
 						process( content, srcpath ) {
-							let matches, version, versionRegex;
-							if ( /amp\.php$/.test( srcpath ) ) {
-								versionRegex = /(\*\s+Version:\s+)(\d+(\.\d+)+-\w+)/;
-
+							if ( /^amp\.php$/.test( srcpath ) ) {
 								// If not a stable build (e.g. 0.7.0-beta), amend the version with the git commit and current timestamp.
-								matches = content.match( versionRegex );
-								if ( matches ) {
-									version = matches[ 2 ] + '-' + versionAppend;
-									console.log( 'Updating version in amp.php to ' + version ); // eslint-disable-line no-console
-									content = content.replace( versionRegex, '$1' + version );
-									content = content.replace( /(define\(\s*'AMP__VERSION',\s*')(.+?)(?=')/, '$1' + version );
+								if ( matches.identifier ) {
+									const pluginVersion = version + matches.identifier + '-' + versionAppend;
+									console.log( 'Updating version in amp.php to ' + pluginVersion ); // eslint-disable-line no-console
+									content = content.replace( versionRegex, '$1' + pluginVersion );
+									content = content.replace( /(define\(\s*'AMP__VERSION',\s*')(.+?)(?=')/, '$1' + pluginVersion );
 								}
 
 								// Remove dev mode code blocks.
 								content = content.replace( /\n\/\/\s*DEV_CODE.+?\n}\n/s, '' );
+
+								if ( 'composer' === process.env.BUILD_TYPE ) {
+									content = content.replace( "require_once AMP__DIR__ . '/vendor/autoload.php';", '' );
+								}
+							} else if ( /^composer\.json$/.test( srcpath ) && 'composer' === process.env.BUILD_TYPE ) {
+								console.log( 'Setting version in composer.json to ' + version ); // eslint-disable-line no-console
+								content = content.replace( /"name": "ampproject\/amp-wp",/, '$&\n  "version": "' + version + '",' );
 							}
+
 							return content;
 						},
 					},
diff --git a/amp.php b/amp.php
index fba64bd65a9..49f61942fe6 100644
--- a/amp.php
+++ b/amp.php
@@ -157,7 +157,7 @@
 unset( $_amp_required_extensions, $_amp_missing_extensions, $_amp_required_constructs, $_amp_missing_classes, $_amp_missing_functions, $_amp_required_extension, $_amp_construct_type, $_amp_construct, $_amp_constructs );
 
 // DEV_CODE. This block of code is removed during the build process.
-if ( ! file_exists( AMP__DIR__ . '/vendor/autoload.php' ) || ! file_exists( AMP__DIR__ . '/vendor/sabberworm/php-css-parser' ) || ! file_exists( AMP__DIR__ . '/assets/js/amp-block-editor.js' ) ) {
+if ( ! file_exists( AMP__DIR__ . '/vendor/autoload.php' ) || ! file_exists( AMP__DIR__ . '/third-party/vendor/scoper-autoload.php' ) || ! file_exists( AMP__DIR__ . '/assets/js/amp-block-editor.js' ) ) {
 	$_amp_load_errors->add(
 		'build_required',
 		sprintf(
@@ -251,6 +251,7 @@ function _amp_incorrect_plugin_slug_admin_notice() {
 }
 
 require_once AMP__DIR__ . '/vendor/autoload.php';
+require_once AMP__DIR__ . '/third-party/vendor/scoper-autoload.php';
 
 register_activation_hook( __FILE__, 'amp_activate' );
 
diff --git a/codecov.yml b/codecov.yml
index 34dc93a6d6f..7ba02126fb7 100644
--- a/codecov.yml
+++ b/codecov.yml
@@ -42,6 +42,8 @@ comment:
 ignore:
   - "/bin"
   - "/back-compat"
+  - "/third-party"
   - "/docs/**/*"
   - ".phpstorm.meta.php"
   - "/qa-tester/**"
+  - "scoper.inc.php"
diff --git a/composer.json b/composer.json
index 0eb084af1d1..9fde39137af 100644
--- a/composer.json
+++ b/composer.json
@@ -50,6 +50,11 @@
   },
   "extra": {
     "downloads": {
+      "php-scoper": {
+        "path": "vendor/bin/php-scoper",
+        "type": "phar",
+        "url": "https://github.com/humbug/php-scoper/releases/download/0.14.0/php-scoper.phar"
+      },
       "phpstan": {
         "path": "vendor/bin/phpstan",
         "type": "phar",
@@ -91,10 +96,22 @@
   "minimum-stability": "dev",
   "prefer-stable": true,
   "scripts": {
+    "post-install-cmd": [
+      "@scope-dependencies"
+    ],
+    "post-update-cmd": [
+      "@scope-dependencies"
+    ],
     "analyze": "if [ -z $TEST_SKIP_PHPSTAN ]; then phpstan --version; phpstan analyze --ansi; fi",
     "pre-commit": [
       "npm run lint:staged"
     ],
-    "prepare-tests": "install-package-tests"
+    "prepare-tests": "install-package-tests",
+    "scope-dependencies": [
+      "php-scoper add-prefix --output-dir=third-party --force --quiet",
+      "echo '{ \"autoload\": { \"classmap\": [\"\"] } }' > third-party/composer.json",
+      "composer dump-autoload --working-dir third-party --no-dev --no-plugins --classmap-authoritative",
+      "sed -i'.bak' -e 's/Composer\\\\Autoload/AmpProject\\\\AmpWP\\\\Composer\\\\Autoload/' third-party/vendor/composer/*.php && rm -rf third-party/vendor/composer/*.php.bak"
+    ]
   }
 }
diff --git a/composer.lock b/composer.lock
index 548fc78c249..bd4d94a86da 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "a6058fda10827da7ccfc7041bdd0c33c",
+    "content-hash": "f73a29684d97e6b2474f5d742dd8e6cc",
     "packages": [
         {
             "name": "ampproject/amp-toolbox",
@@ -12,12 +12,12 @@
             "source": {
                 "type": "git",
                 "url": "https://github.com/ampproject/amp-toolbox-php.git",
-                "reference": "8e1dfacf4f4df8835c6a9ed1d3be8b4a7bf7b92e"
+                "reference": "3c52c66a3e0665d22111d4552e23d61d033ba1d0"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/ampproject/amp-toolbox-php/zipball/8e1dfacf4f4df8835c6a9ed1d3be8b4a7bf7b92e",
-                "reference": "8e1dfacf4f4df8835c6a9ed1d3be8b4a7bf7b92e",
+                "url": "https://api.github.com/repos/ampproject/amp-toolbox-php/zipball/3c52c66a3e0665d22111d4552e23d61d033ba1d0",
+                "reference": "3c52c66a3e0665d22111d4552e23d61d033ba1d0",
                 "shasum": ""
             },
             "require": {
@@ -72,7 +72,7 @@
                 "issues": "https://github.com/ampproject/amp-toolbox-php/issues",
                 "source": "https://github.com/ampproject/amp-toolbox-php/tree/main"
             },
-            "time": "2021-04-25T07:16:15+00:00"
+            "time": "2021-04-28T01:26:05+00:00"
         },
         {
             "name": "cweagans/composer-patches",
@@ -229,7 +229,7 @@
                 "issues": "https://github.com/sabberworm/PHP-CSS-Parser/issues",
                 "source": "https://github.com/sabberworm/PHP-CSS-Parser/tree/master"
             },
-            "time": "2021-04-23T05:58:34+00:00"
+            "time": "2021-04-26T15:40:51+00:00"
         },
         {
             "name": "willwashburn/stream",
@@ -286,16 +286,16 @@
     "packages-dev": [
         {
             "name": "automattic/vipwpcs",
-            "version": "2.3.1",
+            "version": "2.3.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/Automattic/VIP-Coding-Standards.git",
-                "reference": "90173cec6f4df2af9a31627f4880874d788459ab"
+                "reference": "efacebef421334d54b99afa92fb8fa645336a8a7"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/Automattic/VIP-Coding-Standards/zipball/90173cec6f4df2af9a31627f4880874d788459ab",
-                "reference": "90173cec6f4df2af9a31627f4880874d788459ab",
+                "url": "https://api.github.com/repos/Automattic/VIP-Coding-Standards/zipball/efacebef421334d54b99afa92fb8fa645336a8a7",
+                "reference": "efacebef421334d54b99afa92fb8fa645336a8a7",
                 "shasum": ""
             },
             "require": {
@@ -334,7 +334,7 @@
                 "source": "https://github.com/Automattic/VIP-Coding-Standards",
                 "wiki": "https://github.com/Automattic/VIP-Coding-Standards/wiki"
             },
-            "time": "2021-04-23T14:52:17+00:00"
+            "time": "2021-04-28T16:41:50+00:00"
         },
         {
             "name": "behat/behat",
@@ -1199,16 +1199,16 @@
         },
         {
             "name": "guzzlehttp/psr7",
-            "version": "1.8.1",
+            "version": "1.8.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/psr7.git",
-                "reference": "35ea11d335fd638b5882ff1725228b3d35496ab1"
+                "reference": "dc960a912984efb74d0a90222870c72c87f10c91"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/psr7/zipball/35ea11d335fd638b5882ff1725228b3d35496ab1",
-                "reference": "35ea11d335fd638b5882ff1725228b3d35496ab1",
+                "url": "https://api.github.com/repos/guzzle/psr7/zipball/dc960a912984efb74d0a90222870c72c87f10c91",
+                "reference": "dc960a912984efb74d0a90222870c72c87f10c91",
                 "shasum": ""
             },
             "require": {
@@ -1268,9 +1268,9 @@
             ],
             "support": {
                 "issues": "https://github.com/guzzle/psr7/issues",
-                "source": "https://github.com/guzzle/psr7/tree/1.8.1"
+                "source": "https://github.com/guzzle/psr7/tree/1.8.2"
             },
-            "time": "2021-03-21T16:25:00+00:00"
+            "time": "2021-04-26T09:17:50+00:00"
         },
         {
             "name": "mikey179/vfsstream",
@@ -2931,23 +2931,30 @@
         },
         {
             "name": "rmccue/requests",
-            "version": "v1.7.0",
+            "version": "v1.8.0",
             "source": {
                 "type": "git",
-                "url": "https://github.com/rmccue/Requests.git",
-                "reference": "87932f52ffad70504d93f04f15690cf16a089546"
+                "url": "https://github.com/WordPress/Requests.git",
+                "reference": "afbe4790e4def03581c4a0963a1e8aa01f6030f1"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/rmccue/Requests/zipball/87932f52ffad70504d93f04f15690cf16a089546",
-                "reference": "87932f52ffad70504d93f04f15690cf16a089546",
+                "url": "https://api.github.com/repos/WordPress/Requests/zipball/afbe4790e4def03581c4a0963a1e8aa01f6030f1",
+                "reference": "afbe4790e4def03581c4a0963a1e8aa01f6030f1",
                 "shasum": ""
             },
             "require": {
                 "php": ">=5.2"
             },
             "require-dev": {
-                "requests/test-server": "dev-master"
+                "dealerdirect/phpcodesniffer-composer-installer": "^0.7",
+                "php-parallel-lint/php-console-highlighter": "^0.5.0",
+                "php-parallel-lint/php-parallel-lint": "^1.3",
+                "phpcompatibility/php-compatibility": "^9.0",
+                "phpunit/phpunit": "^4.8 || ^5.7 || ^6.5 || ^7.5",
+                "requests/test-server": "dev-master",
+                "squizlabs/php_codesniffer": "^3.5",
+                "wp-coding-standards/wpcs": "^2.0"
             },
             "type": "library",
             "autoload": {
@@ -2966,7 +2973,7 @@
                 }
             ],
             "description": "A HTTP library written in PHP, for human beings.",
-            "homepage": "http://github.com/rmccue/Requests",
+            "homepage": "http://github.com/WordPress/Requests",
             "keywords": [
                 "curl",
                 "fsockopen",
@@ -2977,10 +2984,10 @@
                 "sockets"
             ],
             "support": {
-                "issues": "https://github.com/rmccue/Requests/issues",
-                "source": "https://github.com/rmccue/Requests/tree/master"
+                "issues": "https://github.com/WordPress/Requests/issues",
+                "source": "https://github.com/WordPress/Requests/tree/v1.8.0"
             },
-            "time": "2016-10-13T00:11:37+00:00"
+            "time": "2021-04-27T11:05:25+00:00"
         },
         {
             "name": "roave/security-advisories",
@@ -2988,12 +2995,12 @@
             "source": {
                 "type": "git",
                 "url": "https://github.com/Roave/SecurityAdvisories.git",
-                "reference": "3c97c13698c448fdbbda20acb871884a2d8f45b1"
+                "reference": "a9d356baedee32afbb2179a2a61a45b215f8c95e"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/Roave/SecurityAdvisories/zipball/3c97c13698c448fdbbda20acb871884a2d8f45b1",
-                "reference": "3c97c13698c448fdbbda20acb871884a2d8f45b1",
+                "url": "https://api.github.com/repos/Roave/SecurityAdvisories/zipball/a9d356baedee32afbb2179a2a61a45b215f8c95e",
+                "reference": "a9d356baedee32afbb2179a2a61a45b215f8c95e",
                 "shasum": ""
             },
             "conflict": {
@@ -3021,7 +3028,7 @@
                 "centreon/centreon": "<18.10.8|>=19,<19.4.5",
                 "cesnet/simplesamlphp-module-proxystatistics": "<3.1",
                 "codeigniter/framework": "<=3.0.6",
-                "composer/composer": "<=1-alpha.11",
+                "composer/composer": "<1.10.22|>=2-alpha.1,<2.0.13",
                 "contao-components/mediaelement": ">=2.14.2,<2.21.1",
                 "contao/core": ">=2,<3.5.39",
                 "contao/core-bundle": ">=4,<4.4.52|>=4.5,<4.9.6|= 4.10.0",
@@ -3133,7 +3140,7 @@
                 "paragonie/random_compat": "<2",
                 "passbolt/passbolt_api": "<2.11",
                 "paypal/merchant-sdk-php": "<3.12",
-                "pear/archive_tar": "<1.4.13",
+                "pear/archive_tar": "<1.4.12",
                 "personnummer/personnummer": "<3.0.2",
                 "phpfastcache/phpfastcache": ">=5,<5.0.13",
                 "phpmailer/phpmailer": "<6.1.6",
@@ -3161,6 +3168,7 @@
                 "pusher/pusher-php-server": "<2.2.1",
                 "pwweb/laravel-core": "<=0.3.6-beta",
                 "rainlab/debugbar-plugin": "<3.1",
+                "rmccue/requests": ">=1.6,<1.8",
                 "robrichards/xmlseclibs": "<3.0.4",
                 "sabberworm/php-css-parser": ">=1,<1.0.1|>=2,<2.0.1|>=3,<3.0.1|>=4,<4.0.1|>=5,<5.0.9|>=5.1,<5.1.3|>=5.2,<5.2.1|>=6,<6.0.2|>=7,<7.0.4|>=8,<8.0.1|>=8.1,<8.1.1|>=8.2,<8.2.1|>=8.3,<8.3.1",
                 "sabre/dav": ">=1.6,<1.6.99|>=1.7,<1.7.11|>=1.8,<1.8.9",
@@ -3326,7 +3334,7 @@
                     "type": "tidelift"
                 }
             ],
-            "time": "2021-04-22T17:19:04+00:00"
+            "time": "2021-04-28T18:10:37+00:00"
         },
         {
             "name": "sebastian/code-unit-reverse-lookup",
diff --git a/includes/options/class-amp-options-manager.php b/includes/options/class-amp-options-manager.php
index 21bd8eb7a40..1698fb07dee 100644
--- a/includes/options/class-amp-options-manager.php
+++ b/includes/options/class-amp-options-manager.php
@@ -43,7 +43,6 @@ class AMP_Options_Manager {
 	 * Sets up hooks.
 	 */
 	public static function init() {
-		add_action( 'admin_notices', [ __CLASS__, 'render_php_css_parser_conflict_notice' ] );
 		add_action( 'admin_notices', [ __CLASS__, 'insecure_connection_notice' ] );
 		add_action( 'admin_notices', [ __CLASS__, 'reader_theme_fallback_notice' ] );
 	}
@@ -421,48 +420,6 @@ public static function update_options( $options ) {
 		return update_option( self::OPTION_NAME, $amp_options, false );
 	}
 
-	/**
-	 * Render PHP-CSS-Parser conflict notice.
-	 *
-	 * @return void
-	 */
-	public static function render_php_css_parser_conflict_notice() {
-		$current_screen = get_current_screen();
-		if ( ! ( $current_screen instanceof WP_Screen ) || 'toplevel_page_' . self::OPTION_NAME !== $current_screen->id ) {
-			return;
-		}
-
-		if ( AMP_Style_Sanitizer::has_required_php_css_parser() ) {
-			return;
-		}
-
-		try {
-			$reflection = new ReflectionClass( 'Sabberworm\CSS\CSSList\CSSList' );
-			$source_dir = str_replace(
-				trailingslashit( WP_CONTENT_DIR ),
-				'',
-				preg_replace( '#/vendor/sabberworm/.+#', '', $reflection->getFileName() )
-			);
-
-			printf(
-				'<div class="notice notice-warning"><p>%s</p></div>',
-				wp_kses(
-					sprintf(
-						/* translators: %s: path to the conflicting library */
-						__( 'A conflicting version of PHP-CSS-Parser appears to be installed by another plugin or theme (located in %s). Because of this, CSS processing will be limited, and tree shaking will not be available.', 'amp' ),
-						'<code>' . esc_html( $source_dir ) . '</code>'
-					),
-					[ 'code' => [] ]
-				)
-			);
-		} catch ( ReflectionException $e ) {
-			printf(
-				'<div class="notice notice-warning"><p>%s</p></div>',
-				esc_html__( 'PHP-CSS-Parser is not available so CSS processing will not be available.', 'amp' )
-			);
-		}
-	}
-
 	/**
 	 * Outputs an admin notice if the site is not served over HTTPS.
 	 *
diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 28658d89112..d005fbbced6 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -13,21 +13,24 @@
 use AmpProject\Dom\Document;
 use AmpProject\Exception\FailedToGetFromRemoteUrl;
 use AmpProject\RemoteGetRequest;
-use Sabberworm\CSS\RuleSet\DeclarationBlock;
-use Sabberworm\CSS\CSSList\CSSList;
-use Sabberworm\CSS\Property\Selector;
-use Sabberworm\CSS\RuleSet\RuleSet;
-use Sabberworm\CSS\Property\AtRule;
-use Sabberworm\CSS\Rule\Rule;
-use Sabberworm\CSS\CSSList\KeyFrame;
-use Sabberworm\CSS\RuleSet\AtRuleSet;
-use Sabberworm\CSS\OutputFormat;
-use Sabberworm\CSS\Property\Import;
-use Sabberworm\CSS\CSSList\AtRuleBlockList;
-use Sabberworm\CSS\Value\RuleValueList;
-use Sabberworm\CSS\Value\URL;
-use Sabberworm\CSS\Value\Value;
-use Sabberworm\CSS\CSSList\Document as CSSDocument;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\RuleSet\DeclarationBlock;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\CSSList\CSSList;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Property\Selector;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\RuleSet\RuleSet;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Property\AtRule;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Rule\Rule;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\CSSList\KeyFrame;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\RuleSet\AtRuleSet;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\OutputFormat;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Property\Import;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\CSSList\AtRuleBlockList;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Value\RuleValueList;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Value\URL;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Value\Value;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\CSSList\Document as CSSDocument;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Settings as CSSSettings;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\Parser as CSSParser;
+use AmpProject\AmpWP\ScopedDependencies\Sabberworm\CSS\OutputFormat as CSSOutputFormat;
 
 /**
  * Class AMP_Style_Sanitizer
@@ -376,6 +379,8 @@ public static function get_css_parser_validation_error_codes() {
 	/**
 	 * Determine whether the version of PHP-CSS-Parser loaded has all required features for tree shaking and CSS processing.
 	 *
+	 * @codeCoverageIgnore
+	 * @deprecated As of v2.1 a patched version of PHP-CSS-Parser is now bundled with the plugin.
 	 * @since 1.0.2
 	 *
 	 * @return bool Returns true if the plugin's forked version of PHP-CSS-Parser is loaded by Composer.
@@ -1803,8 +1808,8 @@ private function create_validated_css_document( $stylesheet_string, $options ) {
 			// Remove spaces from data URLs, which cause errors and PHP-CSS-Parser can't handle them.
 			$stylesheet_string = $this->remove_spaces_from_url_values( $stylesheet_string );
 
-			$parser_settings = Sabberworm\CSS\Settings::create();
-			$css_parser      = new Sabberworm\CSS\Parser( $stylesheet_string, $parser_settings );
+			$parser_settings = CSSSettings::create();
+			$css_parser      = new CSSParser( $stylesheet_string, $parser_settings );
 			$css_document    = $css_parser->parse(); // @todo If 'utf-8' is not $css_parser->getCharset() then issue warning?
 
 			if ( ! empty( $options['stylesheet_url'] ) ) {
@@ -1904,7 +1909,7 @@ private function parse_stylesheet( $stylesheet_string, $options = [] ) {
 		if ( ! empty( $parsed_stylesheet['css_document'] ) ) {
 			$css_document = $parsed_stylesheet['css_document'];
 
-			$output_format = Sabberworm\CSS\OutputFormat::createCompact();
+			$output_format = CSSOutputFormat::createCompact();
 			$output_format->setSemicolonAfterLastRule( false );
 
 			$before_declaration_block          = sprintf( '/*%s*/', chr( 1 ) );
@@ -1915,15 +1920,12 @@ private function parse_stylesheet( $stylesheet_string, $options = [] ) {
 			$before_at_rule                    = sprintf( '/*%s*/', chr( 6 ) );
 			$after_at_rule                     = sprintf( '/*%s*/', chr( 7 ) );
 
-			// Add comments to stylesheet if PHP-CSS-Parser has the required extensions for tree shaking.
-			if ( self::has_required_php_css_parser() ) {
-				$output_format->set( 'BeforeDeclarationBlock', $before_declaration_block );
-				$output_format->set( 'SpaceBeforeSelectorSeparator', $between_selectors );
-				$output_format->set( 'AfterDeclarationBlockSelectors', $after_declaration_block_selectors );
-				$output_format->set( 'AfterDeclarationBlock', $after_declaration_block );
-				$output_format->set( 'BeforeAtRuleBlock', $before_at_rule );
-				$output_format->set( 'AfterAtRuleBlock', $after_at_rule );
-			}
+			$output_format->set( 'BeforeDeclarationBlock', $before_declaration_block );
+			$output_format->set( 'SpaceBeforeSelectorSeparator', $between_selectors );
+			$output_format->set( 'AfterDeclarationBlockSelectors', $after_declaration_block_selectors );
+			$output_format->set( 'AfterDeclarationBlock', $after_declaration_block );
+			$output_format->set( 'BeforeAtRuleBlock', $before_at_rule );
+			$output_format->set( 'AfterAtRuleBlock', $after_at_rule );
 			$output_format->set( 'SpaceBetweenRules', $between_properties );
 
 			$stylesheet_string = $css_document->render( $output_format );
diff --git a/includes/validation/class-amp-validated-url-post-type.php b/includes/validation/class-amp-validated-url-post-type.php
index 9284334d7eb..fc076295564 100644
--- a/includes/validation/class-amp-validated-url-post-type.php
+++ b/includes/validation/class-amp-validated-url-post-type.php
@@ -2341,14 +2341,6 @@ static function ( $a, $b ) use ( $stylesheets ) {
 
 		?>
 
-		<?php if ( ! AMP_Style_Sanitizer::has_required_php_css_parser() ) : ?>
-			<div class="notice notice-alt notice-warning inline">
-				<p>
-					<?php esc_html_e( 'AMP CSS processing is limited because a conflicting version of PHP-CSS-Parser has been loaded by another plugin or theme. Tree shaking is not available.', 'amp' ); ?>
-				</p>
-			</div>
-		<?php endif; ?>
-
 		<table class="amp-stylesheet-summary">
 			<tr>
 				<th>
diff --git a/package.json b/package.json
index 7b7d9a95f18..3c98ca8eb7d 100644
--- a/package.json
+++ b/package.json
@@ -94,8 +94,9 @@
     "webpackbar": "4.0.0"
   },
   "scripts": {
-    "build:dev": "cross-env NODE_ENV=development npm-run-all 'build:!(dev|prod)'",
-    "build:prod": "cross-env NODE_ENV=production npm-run-all 'build:!(dev|prod)'",
+    "build:dev": "cross-env NODE_ENV=development npm-run-all 'build:!(dev|prod|composer)'",
+    "build:prod": "cross-env NODE_ENV=production npm-run-all 'build:!(dev|prod|composer)'",
+    "build:composer": "cross-env NODE_ENV=production BUILD_TYPE=composer npm-run-all 'build:!(dev|prod|composer)'",
     "build:prepare": "grunt clean",
     "build:js": "wp-scripts build",
     "build:run": "grunt build",
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 210521e26d4..7977834706e 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -26,6 +26,7 @@ parameters:
 		- %currentWorkingDirectory%/tests/php/static-analysis-stubs/twentyseventeen.php
 		- %currentWorkingDirectory%/tests/php/static-analysis-stubs/legacy-i18n.php
 		- %currentWorkingDirectory%/vendor/autoload.php
+		- %currentWorkingDirectory%/third-party/vendor/scoper-autoload.php
 		- %currentWorkingDirectory%/amp.php
 		- %currentWorkingDirectory%/includes/amp-frontend-actions.php
 		- %currentWorkingDirectory%/includes/amp-post-template-functions.php
diff --git a/scoper.inc.php b/scoper.inc.php
new file mode 100644
index 00000000000..b49fc4dfd4c
--- /dev/null
+++ b/scoper.inc.php
@@ -0,0 +1,73 @@
+<?php
+
+use Isolated\Symfony\Component\Finder\Finder;
+
+return [
+	// The prefix configuration. If a non null value will be used, a random prefix will be generated.
+	'prefix'                     => 'AmpProject\\AmpWP\\ScopedDependencies',
+
+	// By default when running php-scoper add-prefix, it will prefix all relevant code found in the current working
+	// directory. You can however define which files should be scoped by defining a collection of Finders in the
+	// following configuration key.
+	//
+	// For more see: https://github.com/humbug/php-scoper#finders-and-paths.
+	'finders'                    => [
+		Finder::create()
+			->files()
+			->ignoreVCS( true )
+			->ignoreDotFiles( true )
+			->name( '*.php' )
+			->exclude( [ 'tests' ] )
+			->in( 'vendor/sabberworm/php-css-parser' )
+			->append( [ 'vendor/sabberworm/php-css-parser/composer.json' ] ),
+
+		// Main composer.json file so that we can build a classmap.
+		Finder::create()
+			->append( [ 'composer.json' ] ),
+	],
+
+	// Whitelists a list of files. Unlike the other whitelist related features, this one is about completely leaving
+	// a file untouched.
+	// Paths are relative to the configuration file unless if they are already absolute.
+	'files-whitelist'            => [],
+
+	// When scoping PHP files, there will be scenarios where some of the code being scoped indirectly references the
+	// original namespace. These will include, for example, strings or string manipulations. PHP-Scoper has limited
+	// support for prefixing such strings. To circumvent that, you can define patchers to manipulate the file to your
+	// heart contents.
+	//
+	// For more see: https://github.com/humbug/php-scoper#patchers.
+	'patchers'                   => [
+		function ( $filePath, $prefix, $contents ) { //phpcs:ignore WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase
+			// Change the contents here.
+
+			return $contents;
+		},
+	],
+
+	// PHP-Scoper's goal is to make sure that all code for a project lies in a distinct PHP namespace. However, you
+	// may want to share a common API between the bundled code of your PHAR and the consumer code. For example if
+	// you have a PHPUnit PHAR with isolated code, you still want the PHAR to be able to understand the
+	// PHPUnit\Framework\TestCase class.
+	//
+	// A way to achieve this is by specifying a list of classes to not prefix with the following configuration key. Note
+	// that this does not work with functions or constants neither with classes belonging to the global namespace.
+	//
+	// Fore more see https://github.com/humbug/php-scoper#whitelist.
+	'whitelist'                  => [],
+
+	// If `true` then the user defined constants belonging to the global namespace will not be prefixed.
+	//
+	// For more see https://github.com/humbug/php-scoper#constants--constants--functions-from-the-global-namespace.
+	'whitelist-global-constants' => false,
+
+	// If `true` then the user defined classes belonging to the global namespace will not be prefixed.
+	//
+	// For more see https://github.com/humbug/php-scoper#constants--constants--functions-from-the-global-namespace.
+	'whitelist-global-classes'   => false,
+
+	// If `true` then the user defined functions belonging to the global namespace will not be prefixed.
+	//
+	// For more see https://github.com/humbug/php-scoper#constants--constants--functions-from-the-global-namespace.
+	'whitelist-global-functions' => false,
+];
diff --git a/tests/php/test-class-amp-options-manager.php b/tests/php/test-class-amp-options-manager.php
index 49ec10fe91c..7a46e2f7fbf 100644
--- a/tests/php/test-class-amp-options-manager.php
+++ b/tests/php/test-class-amp-options-manager.php
@@ -73,7 +73,6 @@ public function test_constants() {
 	 */
 	public function test_init() {
 		AMP_Options_Manager::init();
-		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'render_php_css_parser_conflict_notice' ] ) );
 		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'insecure_connection_notice' ] ) );
 		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'reader_theme_fallback_notice' ] ) );
 	}
@@ -711,17 +710,6 @@ public function test_get_options_theme_support_defaults( $args, $expected_mode,
 		$this->assertEquals( $expected_mode, AMP_Options_Manager::get_option( Option::THEME_SUPPORT ) );
 	}
 
-	/** @covers AMP_Options_Manager::render_php_css_parser_conflict_notice() */
-	public function test_render_php_css_parser_conflict_notice() {
-		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
-
-		set_current_screen( 'themes' );
-		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
-
-		set_current_screen( 'toplevel_page_' . AMP_Options_Manager::OPTION_NAME );
-		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
-	}
-
 	/** @covers AMP_Options_Manager::insecure_connection_notice() */
 	public function test_insecure_connection_notice() {
 		$_SERVER['HTTPS'] = false;