diff --git a/extensions/css-language-features/.vscode/launch.json b/extensions/css-language-features/.vscode/launch.json
index f6f6922c78da6efeb7fd01f89bc73adb7b4d4fed..9aad19d5b4e824230094bd84ce62af704c721e18 100644
--- a/extensions/css-language-features/.vscode/launch.json
+++ b/extensions/css-language-features/.vscode/launch.json
@@ -3,7 +3,10 @@
 	"compounds": [
 		{
 			"name": "Debug Extension and Language Server",
-			"configurations": ["Launch Extension", "Attach Language Server"]
+			"configurations": [
+				"Launch Extension",
+				"Attach Language Server"
+			]
 		}
 	],
 	"configurations": [
@@ -17,7 +20,9 @@
 			],
 			"stopOnEntry": false,
 			"sourceMaps": true,
-			"outFiles": ["${workspaceFolder}/client/out/**/*.js"],
+			"outFiles": [
+				"${workspaceFolder}/client/out/**/*.js"
+			],
 			"smartStep": true,
 			"preLaunchTask": "npm: compile"
 		},
@@ -26,10 +31,15 @@
 			"type": "extensionHost",
 			"request": "launch",
 			"runtimeExecutable": "${execPath}",
-			"args": ["--extensionDevelopmentPath=${workspaceFolder}", "--extensionTestsPath=${workspaceFolder}/client/out/test" ],
+			"args": [
+				"--extensionDevelopmentPath=${workspaceFolder}",
+				"--extensionTestsPath=${workspaceFolder}/client/out/test"
+			],
 			"stopOnEntry": false,
 			"sourceMaps": true,
-			"outFiles": ["${workspaceFolder}/client/out/test/**/*.js"],
+			"outFiles": [
+				"${workspaceFolder}/client/out/test/**/*.js"
+			],
 			"preLaunchTask": "npm: compile"
 		},
 		{
@@ -39,7 +49,9 @@
 			"protocol": "inspector",
 			"port": 6044,
 			"sourceMaps": true,
-			"outFiles": ["${workspaceFolder}/server/out/**/*.js"],
+			"outFiles": [
+				"${workspaceFolder}/server/out/**/*.js"
+			],
 			"smartStep": true,
 			"restart": true
 		}
diff --git a/extensions/css-language-features/package.json b/extensions/css-language-features/package.json
index 9ef0c79b3f6a76214058e48a4b24f99965404a9e..9bfc56b2e3f8e028340e1afac0c9740c08998873 100644
--- a/extensions/css-language-features/package.json
+++ b/extensions/css-language-features/package.json
@@ -243,6 +243,17 @@
             "default": "ignore",
             "description": "%css.lint.idSelector.desc%"
           },
+          "css.lint.unknownAtRules": {
+            "type": "string",
+            "scope": "resource",
+            "enum": [
+              "ignore",
+              "warning",
+              "error"
+            ],
+            "default": "warning",
+            "description": "%css.lint.unknownAtRules.desc%"
+          },
           "css.trace.server": {
             "type": "string",
             "scope": "window",
@@ -704,4 +715,4 @@
     "@types/node": "7.0.43",
     "mocha": "^5.2.0"
   }
-}
+}
\ No newline at end of file
diff --git a/extensions/css-language-features/package.nls.json b/extensions/css-language-features/package.nls.json
index cbe77ea0abb55c405477c97492004682913f787f..b4078e7f5cdae63d9b1caed4f55a13660985c9b4 100644
--- a/extensions/css-language-features/package.nls.json
+++ b/extensions/css-language-features/package.nls.json
@@ -16,6 +16,7 @@
 	"css.lint.importStatement.desc": "Import statements do not load in parallel",
 	"css.lint.propertyIgnoredDueToDisplay.desc": "Property is ignored due to the display. E.g. with 'display: inline', the width, height, margin-top, margin-bottom, and float properties have no effect",
 	"css.lint.universalSelector.desc": "The universal selector (*) is known to be slow",
+	"css.lint.unknownAtRules.desc": "Unknown at-rule.",
 	"css.lint.unknownProperties.desc": "Unknown property.",
 	"css.lint.unknownVendorSpecificProperties.desc": "Unknown vendor specific property.",
 	"css.lint.vendorPrefix.desc": "When using a vendor-specific prefix also include the standard property",