Don't get me wrong. I'm all for teaching more people to code. More importantly, getting more people to build stuff. But trying to teach newcomers to something historically bloated like HTML, sounds discouraging. Many others said before, HTML, CSS, JavaScript are pretty much the Machine Language of the web. It made sense for us to learn those first when it was all we had. But moved forward, it seems more intelligent to get more people to build stuff by having them use higher abstraction layers first. Even more, try to develop even higher layers that'll enable even more people to contribute.
I love Mozilla and I love the initiative. But I don't honestly think this will help much.