Index: src/wp-admin/load-scripts.php
===================================================================
--- src/wp-admin/load-scripts.php	(revision 29129)
+++ src/wp-admin/load-scripts.php	(working copy)
@@ -123,38 +123,52 @@
 
 if ( empty($load) )
 	exit;
 
 require(ABSPATH . WPINC . '/script-loader.php');
 require(ABSPATH . WPINC . '/version.php');
 
 $compress = ( isset($_GET['c']) && $_GET['c'] );
 $force_gzip = ( $compress && 'gzip' == $_GET['c'] );
 $expires_offset = 31536000; // 1 year
 $out = '';
 
 $wp_scripts = new WP_Scripts();
 wp_default_scripts($wp_scripts);
 
-foreach( $load as $handle ) {
-	if ( !array_key_exists($handle, $wp_scripts->registered) )
+$etag = "wp:{$wp_version},";
+
+foreach ( $load as $i => $handle ) {
+	if ( !array_key_exists($handle, $wp_scripts->registered) ) {
+		unset( $load[ $i ] );
 		continue;
+	}
+	$etag .= $handle . ':' . $wp_scripts->registered[$handle]->ver . ',';
+}
+$etag = md5( $etag );
+
+if ( isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) && stripslashes( $_SERVER['HTTP_IF_NONE_MATCH'] ) === $etag ) {
+	header('HTTP/1.1 304 Not Modified');
+	exit();
+}
 
+foreach ( $load as $handle ) {
 	$path = ABSPATH . $wp_scripts->registered[$handle]->src;
 	$out .= get_file($path) . "\n";
 }
 
+header("Etag: $etag");
 header('Content-Type: application/x-javascript; charset=UTF-8');
 header('Expires: ' . gmdate( "D, d M Y H:i:s", time() + $expires_offset ) . ' GMT');
 header("Cache-Control: public, max-age=$expires_offset");
 
 if ( $compress && ! ini_get('zlib.output_compression') && 'ob_gzhandler' != ini_get('output_handler') && isset($_SERVER['HTTP_ACCEPT_ENCODING']) ) {
 	header('Vary: Accept-Encoding'); // Handle proxies
 	if ( false !== stripos($_SERVER['HTTP_ACCEPT_ENCODING'], 'deflate') && function_exists('gzdeflate') && ! $force_gzip ) {
 		header('Content-Encoding: deflate');
 		$out = gzdeflate( $out, 3 );
 	} elseif ( false !== stripos($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') && function_exists('gzencode') ) {
 		header('Content-Encoding: gzip');
 		$out = gzencode( $out, 3 );
 	}
 }
 
Index: src/wp-admin/load-styles.php
===================================================================
--- src/wp-admin/load-styles.php	(revision 29129)
+++ src/wp-admin/load-styles.php	(working copy)
@@ -98,54 +98,68 @@
 $load = preg_replace( '/[^a-z0-9,_-]+/i', '', $_GET['load'] );
 $load = array_unique( explode( ',', $load ) );
 
 if ( empty($load) )
 	exit;
 
 $compress = ( isset($_GET['c']) && $_GET['c'] );
 $force_gzip = ( $compress && 'gzip' == $_GET['c'] );
 $rtl = ( isset($_GET['dir']) && 'rtl' == $_GET['dir'] );
 $expires_offset = 31536000; // 1 year
 $out = '';
 
 $wp_styles = new WP_Styles();
 wp_default_styles($wp_styles);
 
-foreach( $load as $handle ) {
-	if ( !array_key_exists($handle, $wp_styles->registered) )
+$etag = "wp:{$wp_version},";
+
+foreach ( $load as $i => $handle ) {
+	if ( !array_key_exists($handle, $wp_styles->registered) ) {
+		unset( $load[ $i ] );
 		continue;
+	}
+	$etag .= $handle . ':' . $wp_styles->registered[$handle]->ver . ',';
+}
+$etag = md5( $etag );
+
+if ( isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) && stripslashes( $_SERVER['HTTP_IF_NONE_MATCH'] ) === $etag ) {
+	header('HTTP/1.1 304 Not Modified');
+	exit();
+}
 
+foreach ( $load as $handle ) {
 	$style = $wp_styles->registered[$handle];
 	$path = ABSPATH . $style->src;
 
 	if ( $rtl && ! empty( $style->extra['rtl'] ) ) {
 		// All default styles have fully independent RTL files.
 		$path = str_replace( '.min.css', '-rtl.min.css', $path );
 	}
 
 	$content = get_file( $path ) . "\n";
 
 	if ( strpos( $style->src, '/' . WPINC . '/css/' ) === 0 ) {
 		$content = str_replace( '../images/', '../' . WPINC . '/images/', $content );
 		$content = str_replace( '../js/tinymce/', '../' . WPINC . '/js/tinymce/', $content );
 		$content = str_replace( '../fonts/', '../' . WPINC . '/fonts/', $content );
 		$out .= $content;
 	} else {
 		$out .= str_replace( '../images/', 'images/', $content );
 	}
 }
 
+header("Etag: $etag");
 header('Content-Type: text/css; charset=UTF-8');
 header('Expires: ' . gmdate( "D, d M Y H:i:s", time() + $expires_offset ) . ' GMT');
 header("Cache-Control: public, max-age=$expires_offset");
 
 if ( $compress && ! ini_get('zlib.output_compression') && 'ob_gzhandler' != ini_get('output_handler') && isset($_SERVER['HTTP_ACCEPT_ENCODING']) ) {
 	header('Vary: Accept-Encoding'); // Handle proxies
 	if ( false !== stripos($_SERVER['HTTP_ACCEPT_ENCODING'], 'deflate') && function_exists('gzdeflate') && ! $force_gzip ) {
 		header('Content-Encoding: deflate');
 		$out = gzdeflate( $out, 3 );
 	} elseif ( false !== stripos($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') && function_exists('gzencode') ) {
 		header('Content-Encoding: gzip');
 		$out = gzencode( $out, 3 );
 	}
 }