diff --git a/tools/lint-all b/tools/lint-all
index 0208b4d93a..6e7ac6b4ba 100755
--- a/tools/lint-all
+++ b/tools/lint-all
@@ -346,6 +346,8 @@ def build_custom_checkers(by_lang):
          'description': "Missing whitespace after : in CSS"},
         {'pattern': '[a-z]{',
          'description': "Missing whitespace before '{' in CSS."},
+        {'pattern': 'https://',
+         'description': "Zulip CSS should have no dependencies on external resources"},
         {'pattern': '^[ ][ ][a-zA-Z0-9]',
          'description': "Incorrect 2-space indentation in CSS",
          'exclude': set(['static/styles/thirdparty-fonts.css']),