diff --git a/mkdocs/tests/build_tests.py b/mkdocs/tests/build_tests.py
index 7d202ec6..3e649ef5 100644
--- a/mkdocs/tests/build_tests.py
+++ b/mkdocs/tests/build_tests.py
@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 
-import sys
 import unittest
 from unittest import mock
 
@@ -180,7 +179,8 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         self.assertEqual(context['extra_css'], ['../../style.css'])
         self.assertEqual(context['extra_javascript'], ['../../script.js'])
 
-    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
+    # TODO: This shouldn't pass on Linux
+    # @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
     def test_context_extra_css_path_warning(self):
         nav_cfg = [
             {'Home': 'index.md'},
@@ -200,8 +200,8 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         self.assertEqual(context['extra_css'], ['assets/style.css'])
         self.assertEqual(
             '\n'.join(cm.output),
-            "WARNING:mkdocs.utils:Path 'assets\\style.css' uses OS-specific separator '\\', "
-            "change it to '/' so it is recognized on other systems.",
+            "WARNING:mkdocs.utils:Path 'assets\\style.css' uses OS-specific separator '\\'. "
+            "That will be unsupported in a future release. Please change it to '/'.",
         )
 
     def test_context_extra_css_js_no_page(self):
diff --git a/mkdocs/tests/utils/utils_tests.py b/mkdocs/tests/utils/utils_tests.py
index 9ea9b382..6e4d4f27 100644
--- a/mkdocs/tests/utils/utils_tests.py
+++ b/mkdocs/tests/utils/utils_tests.py
@@ -5,7 +5,6 @@ import logging
 import os
 import posixpath
 import stat
-import sys
 import unittest
 from unittest import mock
 
@@ -245,7 +244,8 @@ class UtilsTests(unittest.TestCase):
             urls = utils.create_media_urls(expected_results.keys(), page)
             self.assertEqual([v[i] for v in expected_results.values()], urls)
 
-    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
+    # TODO: This shouldn't pass on Linux
+    # @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
     def test_create_media_urls_windows(self):
         expected_results = {
             'local\\windows\\file\\jquery.js': [
diff --git a/mkdocs/utils/__init__.py b/mkdocs/utils/__init__.py
index f4b9c696..c92aa609 100644
--- a/mkdocs/utils/__init__.py
+++ b/mkdocs/utils/__init__.py
@@ -304,12 +304,12 @@ def normalize_url(path: str, page: Optional[Page] = None, base: str = '') -> str
 def _get_norm_url(path: str) -> Tuple[str, bool]:
     if not path:
         path = '.'
-    elif os.sep != '/' and os.sep in path:
+    elif '\\' in path:
         log.warning(
-            f"Path '{path}' uses OS-specific separator '{os.sep}', "
-            f"change it to '/' so it is recognized on other systems."
+            f"Path '{path}' uses OS-specific separator '\\'. "
+            f"That will be unsupported in a future release. Please change it to '/'."
         )
-        path = path.replace(os.sep, '/')
+        path = path.replace('\\', '/')
     # Allow links to be fully qualified URLs
     parsed = urlsplit(path)
     if parsed.scheme or parsed.netloc or path.startswith(('/', '#')):