diff --git a/doc/api/next_api_changes/2018-09-06-AL-HTMLWriter.rst b/doc/api/next_api_changes/2018-09-06-AL-HTMLWriter.rst
new file mode 100644
index 000000000000..dae0bd26943d
--- /dev/null
+++ b/doc/api/next_api_changes/2018-09-06-AL-HTMLWriter.rst
@@ -0,0 +1,4 @@
+The HTMLWriter constructor is more strict
+`````````````````````````````````````````
+It no longer normalizes unknown values of *default_mode* to 'loop', but errors
+out instead.
diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
index 56e45d5865dc..5280601cbd46 100644
--- a/lib/matplotlib/animation.py
+++ b/lib/matplotlib/animation.py
@@ -846,8 +846,8 @@ def __init__(self, fps=30, codec=None, bitrate=None, extra_args=None,
         self._bytes_limit *= 1024 * 1024
 
         if self.default_mode not in ['loop', 'once', 'reflect']:
-            self.default_mode = 'loop'
-            _log.warning("unrecognized default_mode: using 'loop'")
+            raise ValueError(
+                "unrecognized default_mode {!r}".format(self.default_mode))
 
         super().__init__(fps, codec, bitrate, extra_args, metadata)