[html] errors in attributes on missing semicolons and parens. Fixes #15970

extensions/html/server/src/modes/embeddedSupport.ts

@@ -225,7 +225,7 @@ function append(result: string, str: string, n: number): string {
 }
 
 function getAttributeLanguage(attributeName: string): string {
-	let match = attributeName.match(/^(style)|(on\w+)$/i);
+	let match = attributeName.match(/^(style)$|^(on\w+)$/i);
 	if (!match) {
 		return null;
 	}

extensions/html/server/src/test/embedded.test.ts

@@ -106,6 +106,9 @@ suite('HTML Embedded Support', () => {
 		assertLanguageId('<DIV ONKEYUP=foo(|)</DIV>', 'javascript');
 		assertLanguageId('<DIV ONKEYUP=foo()|</DIV>', 'javascript');
 		assertLanguageId('<DIV ONKEYUP=foo()<|/DIV>', 'html');
+
+		assertLanguageId('<label data-content="|Checkbox"/>', 'html');
+		assertLanguageId('<label on="|Checkbox"/>', 'html');
 	});
 
 	test('Script content', function (): any {