bpo-35603: Add a note on difflib table header interpreted as HTML (GH-11439)

Doc/library/difflib.rst

@@ -127,6 +127,10 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
       the next difference highlight at the top of the browser without any leading
       context).
 
+      .. note::
+         *fromdesc* and *todesc* are interpreted as unescaped HTML and should be
+         properly escaped while receiving input from untrusted sources.
+
       .. versionchanged:: 3.5
          *charset* keyword-only argument was added.  The default charset of
          HTML document changed from ``'ISO-8859-1'`` to ``'utf-8'``.