Support for extensibility in HTML 5

In looking at the latest draft of the HTML5 spec, one of the key areas that I felt was a shortcoming was any provision for extensibility of functionality and the tag set. If I have missed it, I'd love to see it highlighted some more.

Specifically, I think it would be useful to rationalize the functionality of technologies such as HTC and XBL in allowing developers to create new tags/components that encapsulate behavior, rendering, and object model.

It would be interesting to also expose core fundamentals such as layout logic, that allows these custom components to measure and arrange children to create new layouts.

I blogged about extensibility and related thoughts at http://www.nikhilk.net/HTML5-Thoughts.aspx and initial comments posted so far seem to agree that extensibility would be a valuable feature of the platform.

I am sending this mail not to propose something in particular, but really to get extensibility as a core concept in the platform and future iterations..

Thanks,
Nikhil Kothari, Architect, Web Developer Platform,
Microsoft Corporation