A filter to prevernt cross-site scripting (XSS) attacks
$ luarocks install xssfilter
XSSFilter is a module for filtering out unsafe tags from (X)HTML.
To avoid having second-guessing how different clients handle strange
HTML input, the filter assumes that the input is subset of valid XML
and returns nil if the input doesn't parse. If the input does parse,
we then traverse the tree and check each element and its attributes
against a configuration table, replacing anything that's not allowed
with a message. There is a default configuration table that shoots
for a balance between security and features, but the client can either
supply their own or modify the default.