The 404 has been mentioned a couple of times today.
The error was reported in 404 Error Reported
This problem has also been mentioned with hints/help given in Help Improve Compatibility with Browser Fallbacks and About the HTML-CSS category (though the first link is easier to see the help)
A lot of times the user community will ask what you’ve done and then use your response to help you get to the correct solution without explicitly telling you what to do since the lesson kind of walks you through it. The sample that you need to fill in has an area where you need to fill in your answer and they give you a hint as to how to fill it in.