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.
Want to write one? Read How to Write an Extension Specification.
- Encrypted Media Extensions
- Media Source Extensions
- The srcset attribute - Use Cases
- The picture Element - Use Cases
- TextTrackRegion API - Use Cases
- Public Identifiers for entity resolution in XHTML
- Form HTTP Extensions - Rationale
- HTML Ruby Markup Extensions - Rationale for double-sided - Rationale for inlining - Use cases - More use cases - Double-sided in a common book - Murata-san's desiderata