This is the repository for the Working Draft of the HTML specification. This repository is managed by the W3C Web Platform Working Group.
The proposed timeline for HTML5.3 is noted in issue #1076
Contributions to the HTML specification are welcome. The documentation linked above will help you get started.
If you have any questions you can file an issue, contact the HTML editors, or join the W3C IRC server and join the #html channel.
The old HTML repo is available for archival purposes.