Create a user.groups module in ResourceLoader, which bundles a CSS and JS page for each usergroup the user is a member of (MediaWiki:Sysop.js, MediaWiki:Autoconfirmed.css, etc). Groups '*' and 'user' are not included.

RELEASE-NOTES

@@ -72,6 +72,8 @@ it from source control: http://www.mediawiki.org/wiki/Download_from_SVN
   (maintenance/fixDoubleRedirects.php)
 * (bug 23315) New body classes to allow easier styling of special pages
 * (bug 27159) Make email confirmation code expiration time configurable
+* CSS/JS for each user group is imported from MediaWiki:Sysop.js, 
+  MediaWiki:Autoconfirmed.css, etc.
 
 === Bug fixes in 1.18 ===
 * (bug 23119) WikiError class and subclasses are now marked as deprecated

includes/AutoLoader.php

@@ -211,6 +211,7 @@ $wgAutoloadLocalClasses = array(
 	'ResourceLoaderFileModule' => 'includes/resourceloader/ResourceLoaderFileModule.php',
 	'ResourceLoaderSiteModule' => 'includes/resourceloader/ResourceLoaderSiteModule.php',
 	'ResourceLoaderUserModule' => 'includes/resourceloader/ResourceLoaderUserModule.php',
+	'ResourceLoaderUserGroupsModule' => 'includes/resourceloader/ResourceLoaderUserGroupsModule.php',
 	'ResourceLoaderUserOptionsModule' => 'includes/resourceloader/ResourceLoaderUserOptionsModule.php',
 	'ResourceLoaderStartUpModule' => 'includes/resourceloader/ResourceLoaderStartUpModule.php',
 	'ReverseChronologicalPager' => 'includes/Pager.php',

includes/OutputPage.php

@@ -2555,28 +2555,29 @@ class OutputPage {
 		// Legacy Scripts
 		$scripts .= "\n" . $this->mScripts;
 
+		$userScripts = array( 'user.options' );
+
 		// Add site JS if enabled
 		if ( $wgUseSiteJs ) {
 			$scripts .= $this->makeResourceLoaderLink( $sk, 'site', ResourceLoaderModule::TYPE_SCRIPTS );
+			if( $wgUser->isLoggedIn() ){
+				$userScripts[] = 'user.groups';
+			}
 		}
 
-		// Add user JS if enabled - trying to load user.options as a bundle if possible
-		$userOptionsAdded = false;
+		// Add user JS if enabled
 		if ( $wgAllowUserJs && $wgUser->isLoggedIn() ) {
 			$action = $wgRequest->getVal( 'action', 'view' );
 			if( $this->mTitle && $this->mTitle->isJsSubpage() && $sk->userCanPreview( $action ) ) {
 				# XXX: additional security check/prompt?
 				$scripts .= Html::inlineScript( "\n" . $wgRequest->getText( 'wpTextbox1' ) . "\n" ) . "\n";
 			} else {
-				$scripts .= $this->makeResourceLoaderLink(
-					$sk, array( 'user', 'user.options' ), ResourceLoaderModule::TYPE_SCRIPTS
-				);
-				$userOptionsAdded = true;
+				# FIXME: this means that User:Me/Common.js doesn't load when previewing
+				# User:Me/Vector.js, and vice versa (bug26283)
+				$userScripts[] = 'user';
 			}
 		}
-		if ( !$userOptionsAdded ) {
-			$scripts .= $this->makeResourceLoaderLink( $sk, 'user.options', ResourceLoaderModule::TYPE_SCRIPTS );
-		}
+		$scripts .= $this->makeResourceLoaderLink( $sk, $userScripts, ResourceLoaderModule::TYPE_SCRIPTS );
 
 		return $scripts;
 	}

includes/Skin.php

@@ -546,7 +546,7 @@ abstract class Skin extends Linker {
 	 * @private
 	 */
 	function setupUserCss( OutputPage $out ) {
-		global $wgRequest;
+		global $wgRequest, $wgUser;
 		global $wgUseSiteCss, $wgAllowUserCss, $wgAllowUserCssPrefs;
 
 		wfProfileIn( __METHOD__ );
@@ -560,6 +560,9 @@ abstract class Skin extends Linker {
 		// Per-site custom styles
 		if ( $wgUseSiteCss ) {
 			$out->addModuleStyles( 'site' );
+			if( $wgUser->isLoggedIn() ){
+				$out->addModuleStyles( 'user.groups' );
+			}
 		}
 
 		// Per-user custom styles

includes/resourceloader/ResourceLoaderUserGroupsModule.php

@@ -0,0 +1,61 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @author Trevor Parscal
+ * @author Roan Kattouw
+ */
+
+/**
+ * Module for user customizations
+ */
+class ResourceLoaderUserGroupsModule extends ResourceLoaderWikiModule {
+
+	/* Protected Methods */
+	protected $origin = self::ORIGIN_USER_SITEWIDE;
+
+	protected function getPages( ResourceLoaderContext $context ) {
+		if ( $context->getUser() ) {
+			$user = User::newFromName( $context->getUser() );
+			if( $user instanceof User ){
+				$pages = array();
+				foreach( $user->getEffectiveGroups() as $group ){
+					if( in_array( $group, array( '*', 'user' ) ) ){
+						continue;
+					}
+					$g = ucfirst( $group );
+					$pages["MediaWiki:$g.js"] = array( 'type' => 'script' );
+					$pages["MediaWiki:$g.css"] = array( 'type' => 'style' );
+				}
+				return $pages;
+			}
+		}
+		return array();
+	}
+	
+	/* Methods */
+	
+	public function getGroup() {
+		return 'user';
+	}
+	
+	public function getFlip( $context ) {
+		global $wgContLang;
+
+		return $wgContLang->getDir() !== $context->getDirection();
+	}
+}

resources/Resources.php

@@ -8,6 +8,7 @@ return array(
 	'startup' => array( 'class' => 'ResourceLoaderStartUpModule' ),
 	'user' => array( 'class' => 'ResourceLoaderUserModule' ),
 	'user.options' => array( 'class' => 'ResourceLoaderUserOptionsModule' ),
+	'user.groups' => array( 'class' => 'ResourceLoaderUserGroupsModule' ),
 
 	/* Skins */