This theme was originally conceived of as a place for a few dozen Drupal front-end developers to work together to figure out how best to get Drupal 7 to do HTML5. We had lots of debates and conversations, and wrote (re-wrote and re-wrote) its code. Several months into the process we decided to use HTML5 Base as a testing-ground to try out ideas for Drupal 8 core — a place get ideas running on real-world websites, and to use as a proof-of-concept for convincing others in the Drupal core community. In that way, the project was a great success — HTML5 became an official initiative for Drupal 8. But the new leadership appointed to oversee HTML5 in Drupal 8 had a different plan for how to proceed. This theme went through an identity crisis, and was basically abandoned.
The HTML5 Tools module (http://drupal.org/project/html5_tools), meanwhile, is alive and well. The Fences module has joined it as an invaluable tool for getting Drupal to output HTML5 (http://drupal.org/project/fences). Use both.
Now almost two years later, there are many Drupal themes available that implement HTML5, each in their own way. We recommend the 7x.-5.x branch of Zen (https://drupal.org/project/zen).
Feel free to use the code that is here — although we recommend you approach it as an unbaked pile of ideas, not as a finished product. It's always been very rough.
If you've already implemented HTML Base on your website, don't worry. There's nothing that will break your site in the future. No need to worry about support. Your site will keep doing what it's doing. You can always make changes to your own theme to implement new ideas that you have — or not. It's not like a module, where you need to have someone keeping things up to date. Your copy of this theme will be fine.