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 +1,17 @@
+#require test-repo hg10
+
+  $ . "$TESTDIR/helpers-testrepo.sh"
+
+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 except vendored dependencies
+
+  $ cd "`dirname "$TESTDIR"`"
+
+  $ testrepohg locate 'set:**.js' \
+  > 2>/dev/null \
+  > | xargs file -i \
+  > | grep -v 'charset=us-ascii'