title: How can I validate HTML input to prevent XSS?
body: <p>For example, StackExchange whitelists a subset of HTML:
  <a href="https://meta.stackexchange.com/questions/1777/what-html-tags-are-allowed-on-stack-exchange-sites">https://meta.stackexchange.com/questions/1777/what-html-tags-are-allowed-on-stack-exchange-sites</a></p>
  
  <p>How could you do that in your controller to make sure user input is safe?</p>