When rendering the markdown code blocks we replace the
offending characters in the output string with spans highlighting a textual
representation of the character, along with a title attribute with
information about why the character was highlighted.
The list of characters stripped by this fix, which are the bidirectional
characters considered relevant, are:
U+202A
U+202B
U+202C
U+202D
U+202E
U+2066
U+2067
U+2068
U+2069