The site introduces the various aspects of coding in HTML5 in a structured, fun way. You build a website on a phone-style console - the instructions are clear and concise. Help is provided (sometimes costs you points). I found that some of my students enjoyed the site so much that they did far more than I needed them to - some even completed the entire 20 lesson HTML/CSS Level 1 course.
Now - not only does the site have a very good series of lessons teaching all of the basics - but for the teacher it has a running record of how your students are progressing through. This image gives one of the summary progress views.
You can see at a glance student progress, plus the "points" they have received as they progress through the various lessons.
All up, CodeAvengers adds a significant resource to anyone looking to learn HTML/CSS and I will certainly be using this site again next year with students at Years 9, 10 and 11. If you are teaching any of this stuff, or even if you want to learn the language of the web yourself - then this is the best resource I have found to date.
And if you are in NZ - support these guys - they are graduates from the Uni of Waikato I think.