HTML pages inside CHM documentation use the following tag to set applied character set:
    <meta http-equiv="Content-Type" content="text/html; charset=iso8859_1" />

`iso8859_1` is neither valid character set according to IANA registry nor is recognized by most browsers (I've checked IE, Firefox and Opera).

This isn't notable for text in English. But, in `Python Standard Library` in section `4.7.1 String Methods` there is a documentation for str.casefold(). This paragraph contains German letter `ß`.
Without defined character set this letter can be displayed wrongly, depending on user's regional settings. In my Russian I see in CHM `Я`. Firefox displays it as `�`.

Fix of charset to `ISO-8859-1` could resolve the problem.

P.S. Use of UTF-8 may be more convenient for preventing future errors.