Here's a quote from http://www.tizen.org:
Tizen provides a robust and flexible environment for application developers, based on HTML5. With HTML5's robust capabilities and cross platform flexibility, it is rapidly becoming the preferred development environment for mobile apps and services. The Tizen SDK and API allow developers to use HTML5 and related web technologies to write applications that run across multiple device segments.This is a new focus; Tizen originally emphasized both C++ and HTML5 development. Tizen is now focusing on it's ability to run HTML5 apps well. If you consider this with the fact that both Firefox OS and Android (with Google' recent launch of Chrome Apps for Android via Apache Cordova) are pushing HTML5, it is becoming clear that HTML5 is the new place to be for mobile development. (And with the rise of Chrome Apps and Mozilla's Open Web Apps, HTML5 is for the desktop as well.)
I believe that this move to HTML5 is in developers' best interest as it encourages multiple OS platforms. We do not want to go back to the days where Windows and to a much lesser extent MacOS were the only viable games in town. A less dominant Android is in Google's best interest as well -- Windows dominance made Microsoft fat and lazy as similar dominance ruined IBM earlier... Google surely doesn't want to follow that route.
When I last took a serious look about a year ago, Tizen looked immature. Take a look at the demos in the links below and you can see things are working quite polished now. Tizen is already being used in Samsung's latest version of their Gear smartwatch. Samsung says phones running Tizen will release this year.
Samsung Tizen OS: 6 Things You Need To Know - Opinion - Trusted Reviews
Hands-on with Samsung's vastly improved Tizen OS
Exploring Samsung's Tizen smartphone: A glance into the future - Pocket-lint
Information on Samsung's new Tizen-based Gear 2 smartwatches:
Smartwatches: Samsung's Galaxy Gear 2 Watch Drops Android for Tizen | TIME.com
MWC 2014: Samsung announces Tizen-powered Gear 2 and Gear 2 Neo smartwatches | ZDNet
Samsung’s Gear Trio – why Tizen won over Android | Android | Geek.com
The Tizen SDK for some of the new watches is already available:
Samsung's new Galaxy Gear watches are now open to third-party support