This used Bootstrap and was based on this UX advice. This site demonstrated the importance of giving the user a clear path when he or she is lost.
I originally wanted to build something amusing like Github’s 404 page
or something incredible like these 40(4) pages. But I learned that a 404 page that appears similar to the rest of the site is less disconcerting for the lost user. Also providing the user the option to contact you with feedback as well as a way to return to the Home page creates a better experience. Overall I learned that sometimes less is more when building a web page.