diff --git a/extensions/html/server/src/htmlServerMain.ts b/extensions/html/server/src/htmlServerMain.ts
index 8081a5ee302..6687ddad854 100644
--- a/extensions/html/server/src/htmlServerMain.ts
+++ b/extensions/html/server/src/htmlServerMain.ts
@@ -23,7 +23,7 @@ import * as nls from 'vscode-nls';
 nls.config(process.env['VSCODE_NLS_CONFIG']);
 
 namespace TagCloseRequest {
-	export const type: RequestType<TextDocumentPositionParams, string, any, any> = new RequestType('html/tag');
+	export const type: RequestType<TextDocumentPositionParams, string | null, any, any> = new RequestType('html/tag');
 }
 
 // Create a connection for the server
diff --git a/extensions/html/server/src/languageModelCache.ts b/extensions/html/server/src/languageModelCache.ts
index b12f77b52a6..7496244c3bb 100644
--- a/extensions/html/server/src/languageModelCache.ts
+++ b/extensions/html/server/src/languageModelCache.ts
@@ -16,7 +16,7 @@ export function getLanguageModelCache<T>(maxEntries: number, cleanupIntervalTime
 	let languageModels: { [uri: string]: { version: number, languageId: string, cTime: number, languageModel: T } } = {};
 	let nModels = 0;
 
-	let cleanupInterval = void 0;
+	let cleanupInterval: NodeJS.Timer | undefined = void 0;
 	if (cleanupIntervalTimeInSec > 0) {
 		cleanupInterval = setInterval(() => {
 			let cutoffTime = Date.now() - cleanupIntervalTimeInSec * 1000;
diff --git a/extensions/html/server/src/modes/cssMode.ts b/extensions/html/server/src/modes/cssMode.ts
index f787b9c2742..79565426a32 100644
--- a/extensions/html/server/src/modes/cssMode.ts
+++ b/extensions/html/server/src/modes/cssMode.ts
@@ -23,7 +23,7 @@ export function getCSSMode(documentRegions: LanguageModelCache<HTMLDocumentRegio
 		configure(options: any) {
 			cssLanguageService.configure(options && options.css);
 		},
-		doValidation(document: TextDocument, settings: Settings) {
+		doValidation(document: TextDocument, settings?: Settings) {
 			let embedded = embeddedCSSDocuments.get(document);
 			return cssLanguageService.doValidation(embedded, cssStylesheets.get(embedded), settings && settings.css);
 		},
diff --git a/extensions/html/server/src/modes/embeddedSupport.ts b/extensions/html/server/src/modes/embeddedSupport.ts
index 82b9c9cf4c1..f4271889fdf 100644
--- a/extensions/html/server/src/modes/embeddedSupport.ts
+++ b/extensions/html/server/src/modes/embeddedSupport.ts
@@ -8,21 +8,21 @@
 import { TextDocument, Position, LanguageService, TokenType, Range } from 'vscode-html-languageservice';
 
 export interface LanguageRange extends Range {
-	languageId: string;
+	languageId: string | undefined;
 	attributeValue?: boolean;
 }
 
 export interface HTMLDocumentRegions {
 	getEmbeddedDocument(languageId: string, ignoreAttributeValues?: boolean): TextDocument;
 	getLanguageRanges(range: Range): LanguageRange[];
-	getLanguageAtPosition(position: Position): string;
+	getLanguageAtPosition(position: Position): string | undefined;
 	getLanguagesInDocument(): string[];
 	getImportedScripts(): string[];
 }
 
 export var CSS_STYLE_RULE = '__';
 
-interface EmbeddedRegion { languageId: string; start: number; end: number; attributeValue?: boolean; }
+interface EmbeddedRegion { languageId: string | undefined; start: number; end: number; attributeValue?: boolean; }
 
 
 export function getDocumentRegions(languageService: LanguageService, document: TextDocument): HTMLDocumentRegions {
@@ -45,7 +45,7 @@ export function getDocumentRegions(languageService: LanguageService, document: T
 				regions.push({ languageId: 'css', start: scanner.getTokenOffset(), end: scanner.getTokenEnd() });
 				break;
 			case TokenType.Script:
-				regions.push({ languageId: languageIdFromType!, start: scanner.getTokenOffset(), end: scanner.getTokenEnd() });
+				regions.push({ languageId: languageIdFromType, start: scanner.getTokenOffset(), end: scanner.getTokenEnd() });
 				break;
 			case TokenType.AttributeName:
 				lastAttributeName = scanner.getTokenText();
@@ -146,7 +146,7 @@ function getLanguagesInDocument(document: TextDocument, regions: EmbeddedRegion[
 	return result;
 }
 
-function getLanguageAtPosition(document: TextDocument, regions: EmbeddedRegion[], position: Position): string {
+function getLanguageAtPosition(document: TextDocument, regions: EmbeddedRegion[], position: Position): string | undefined {
 	let offset = document.offsetAt(position);
 	for (let region of regions) {
 		if (region.start <= offset) {
diff --git a/extensions/html/server/src/modes/formatting.ts b/extensions/html/server/src/modes/formatting.ts
index 7fc5f222699..4bd3469ac95 100644
--- a/extensions/html/server/src/modes/formatting.ts
+++ b/extensions/html/server/src/modes/formatting.ts
@@ -6,11 +6,11 @@
 
 import { applyEdits } from '../utils/edits';
 import { TextDocument, Range, TextEdit, FormattingOptions, Position } from 'vscode-languageserver-types';
-import { LanguageModes, Settings } from './languageModes';
+import { LanguageModes, Settings, LanguageModeRange } from './languageModes';
 import { pushAll } from '../utils/arrays';
 import { isEOL } from '../utils/strings';
 
-export function format(languageModes: LanguageModes, document: TextDocument, formatRange: Range, formattingOptions: FormattingOptions, settings: Settings, enabledModes: { [mode: string]: boolean }) {
+export function format(languageModes: LanguageModes, document: TextDocument, formatRange: Range, formattingOptions: FormattingOptions, settings: Settings | undefined, enabledModes: { [mode: string]: boolean }) {
 	let result: TextEdit[] = [];
 
 	let endPos = formatRange.end;
@@ -37,9 +37,11 @@ export function format(languageModes: LanguageModes, document: TextDocument, for
 	let allRanges = languageModes.getModesInRange(document, formatRange);
 	let i = 0;
 	let startPos = formatRange.start;
-	while (i < allRanges.length && allRanges[i].mode.getId() !== 'html') {
+	let isHTML = (range: LanguageModeRange) => range.mode && range.mode.getId() === 'html';
+
+	while (i < allRanges.length && !isHTML(allRanges[i])) {
 		let range = allRanges[i];
-		if (!range.attributeValue && range.mode.format) {
+		if (!range.attributeValue && range.mode && range.mode.format) {
 			let edits = range.mode.format(document, Range.create(startPos, range.end), formattingOptions, settings);
 			pushAll(result, edits);
 		}
diff --git a/extensions/html/server/src/modes/htmlMode.ts b/extensions/html/server/src/modes/htmlMode.ts
index e45aea8bc11..c0146423d67 100644
--- a/extensions/html/server/src/modes/htmlMode.ts
+++ b/extensions/html/server/src/modes/htmlMode.ts
@@ -60,6 +60,7 @@ export function getHTMLMode(htmlLanguageService: HTMLLanguageService): LanguageM
 			if (offset > 0 && text.charAt(offset - 1).match(/[>\/]/g)) {
 				return htmlLanguageService.doTagComplete(document, position, htmlDocuments.get(document));
 			}
+			return null;
 		},
 		onDocumentRemoved(document: TextDocument) {
 			htmlDocuments.onDocumentRemoved(document);
diff --git a/extensions/html/server/src/modes/languageModes.ts b/extensions/html/server/src/modes/languageModes.ts
index e4d420bf1f6..d1a461a1648 100644
--- a/extensions/html/server/src/modes/languageModes.ts
+++ b/extensions/html/server/src/modes/languageModes.ts
@@ -43,7 +43,7 @@ export interface LanguageMode {
 	findDocumentLinks?: (document: TextDocument, documentContext: DocumentContext) => DocumentLink[];
 	findDefinition?: (document: TextDocument, position: Position) => Definition | null;
 	findReferences?: (document: TextDocument, position: Position) => Location[];
-	format?: (document: TextDocument, range: Range, options: FormattingOptions, settings: Settings) => TextEdit[];
+	format?: (document: TextDocument, range: Range, options: FormattingOptions, settings?: Settings) => TextEdit[];
 	findDocumentColors?: (document: TextDocument) => ColorInformation[];
 	getColorPresentations?: (document: TextDocument, color: Color, range: Range) => ColorPresentation[];
 	doAutoClose?: (document: TextDocument, position: Position) => string | null;
@@ -62,7 +62,7 @@ export interface LanguageModes {
 }
 
 export interface LanguageModeRange extends Range {
-	mode: LanguageMode;
+	mode: LanguageMode | undefined;
 	attributeValue?: boolean;
 }
 
@@ -92,10 +92,10 @@ export function getLanguageModes(supportedLanguages: { [languageId: string]: boo
 		},
 		getModesInRange(document: TextDocument, range: Range): LanguageModeRange[] {
 			return documentRegions.get(document).getLanguageRanges(range).map(r => {
-				return {
+				return <LanguageModeRange>{
 					start: r.start,
 					end: r.end,
-					mode: modes[r.languageId],
+					mode: r.languageId && modes[r.languageId],
 					attributeValue: r.attributeValue
 				};
 			});