diff --git a/src/calibre/gui2/viewer/web_view.py b/src/calibre/gui2/viewer/web_view.py
index fc077f1ea5..00b5a253d8 100644
--- a/src/calibre/gui2/viewer/web_view.py
+++ b/src/calibre/gui2/viewer/web_view.py
@@ -149,6 +149,8 @@ class UrlSchemeHandler(QWebEngineUrlSchemeHandler):
                     'application/x-font-truetype':'application/x-font-ttf',
                     'application/font-sfnt': 'application/x-font-ttf',
                 }.get(mime_type, mime_type)
+                if mime_type == 'text/css':
+                    mime_type += '; charset=utf-8'
                 send_reply(rq, mime_type, data)
             except Exception:
                 import traceback