I was intrigued to see what CSS selector engine it was using...

https://github.com/chriso/node.io uses https://github.com/harryf/node-soupselect

https://github.com/harryf/node-soupselect is a port of my https://github.com/simonw/soupselect library for Python

https://github.com/simonw/soupselect is a port of my getElementsBySelector function for JavaScript: http://simonwillison.net/2003/Mar/25/getElementsBySelector/

I'm always surprised to see that code still being used - it's the least complete selector library out there by a long way.