From HTML WG Wiki
The following are a list of extension specifications that are actively being developed. Some of these may be integrated during the Candidate Recommdation phase of HTML 5.0, others may integrate later, and some may proceed independently indefinitely.
- Encrypted Media Extensions
- Media Source Extensions
- HTML Responsive Images Extension
- The srcset attribute
- <subline> - change proposal
- <outlinemask> - change proposal