...quoteMode set to UNESCAPED..
I have enabled quoteMode support in the HTMLEncoder (branch quoteMode_html_filter). That part is working fine and the default is to keep the current behavior of escaping double quotes. But if the option UNESCAPE is set the double quotes in the attributes will not be escaped.
The current workaround is to set the quoteMode to ALL or NUMERIC_SINGLE_QUOTES.
I assume this is also an issue for HTML5Filter.