Thanks for the tests. I left a couple comments.

About encoding quotes: Personally I don’t see much value unless you are encoding an attribute value, in which case I would prefer to use xml.sax.saxutils.quoteattr(). Encoded quotes would only become useful if the “error_message_format” attribute was modified.

A more practical downside is that if “error_content_type” is set to say text/plain, we are adding two somewhat common characters that will get messed up. E.g. the “explain” string for 429 Too Many Requests will include the double-quoted "rate limiting". And an apostrophe could easily be given in a custom error message, e.g. “Can't write a clean error message”.