diff --git a/src/runtime.ts b/src/runtime.ts
index 4614cb3062..eb097c523f 100644
--- a/src/runtime.ts
+++ b/src/runtime.ts
@@ -173,6 +173,7 @@ async function load(config: RuntimeConfig, rawPathname: string | undefined): Pro
 
     return {
       statusCode: 200,
+      contentType: 'text/html; charset=utf-8',
       contents: html,
     };
   } catch (err) {