diff --git a/src/mfm/parse/core/syntax-highlighter.ts b/src/mfm/parse/core/syntax-highlighter.ts
index 2b13608d2..c5157ca1d 100644
--- a/src/mfm/parse/core/syntax-highlighter.ts
+++ b/src/mfm/parse/core/syntax-highlighter.ts
@@ -1,3 +1,5 @@
+import { capitalize } from "../../../prelude/string";
+
 function escape(text: string) {
 	return text
 		.replace(/>/g, '>')
@@ -89,7 +91,7 @@ const _keywords = [
 ];
 
 const keywords = _keywords
-	.concat(_keywords.map(k => k[0].toUpperCase() + k.substr(1)))
+	.concat(_keywords.map(capitalize))
 	.concat(_keywords.map(k => k.toUpperCase()))
 	.sort((a, b) => b.length - a.length);
 
diff --git a/src/prelude/string.ts b/src/prelude/string.ts
new file mode 100644
index 000000000..2b89304f1
--- /dev/null
+++ b/src/prelude/string.ts
@@ -0,0 +1,3 @@
+export function capitalize(s: string): string {
+	return s.charAt(0).toUpperCase() + s.slice(1).toLowerCase();
+}