Chrome extension to delay access to certain web pages?

As a learning project, I'd like to create a Chrome extension that will make you wait for 15 seconds (with some kind of message) before serving up a page that is on a list of time-wasting pages that I've set up (say,, etc).

This seems like it should be possible and somewhat straightforward, but I'm not experienced with this kind of development. Any gotchas to watch out for? Specific javascript areas to learn first?