rgrove/sanitize: Ruby HTML and CSS sanitizer.

Sanitize is an allowlist-based HTML and CSS sanitizer. It removes all HTML and/or CSS from a string except the elements, attributes, and properties you choose to allow. Using a simple configuration syntax, you can tell Sanitize to allow certain HTML elements, certain attributes within those elements, and even certain URL protocols within attributes that contain URLs. You can also allow specific CSS properties, @ rules, and URL protocols in elements or attributes containing CSS. Any HTML or CSS that you don't explicitly allow will be removed. Sanitize is based on the Nokogiri HTML5 parser, which parses HTML the same way modern browsers do, and Crass, which parses CSS the same way modern browsers do. As long as your allowlist config only allows safe markup and CSS, even the most malformed or malicious input will be transformed into safe output.