diff --git a/tests/test-check-encoding.t b/tests/test-check-encoding.t
new file mode 100644
--- /dev/null
+++ b/tests/test-check-encoding.t
@@ -0,0 +1,26 @@
+#require test-repo hg10
+
+  $ . "$TESTDIR/helpers-testrepo.sh"
+
+  $ cat > $TESTTMP/check_ascii.py <<EOF
+  > import sys
+  > for file_path in sys.argv[1:]:
+  >     with open(file_path, 'br') as f:
+  >         try:
+  >             f.read().decode('ascii', 'strict')
+  >         except UnicodeDecodeError as exc:
+  >             print('%s: %s' % (file_path, exc))
+  > EOF
+
+There are some web servers in the wild that can serve static files with an
+incorrect encoding (e.g. https://bz.mercurial-scm.org/show_bug.cgi?id=6559).
+One way to prevent any issues is to not use any non-ASCII characters, e.g.
+URL-encoding them or using HTML entities.
+
+check charset of all tracked files ending in .js
+
+  $ cd "`dirname "$TESTDIR"`"
+
+  $ testrepohg locate 'set:**.js' \
+  > 2>/dev/null \
+  > | xargs "$PYTHON" $TESTTMP/check_ascii.py