Internationalization Tag Set (ITS) is an extension of HTML markup which defines additional attributes and elements that can be used to improve suitability of web page for localization, internationalization and automatic translation.
Explore our ITS docs
For human fridendly introduction to ITS please watch the following video http://www.youtube.com/watch?v=5Goet3hX6Jo
List of tools supporting ITS is available at http://www.w3.org/International/its/wiki/ITS_Implementations
You can check whether your page with embedded ITS markup is valid by using Nu Markup Validation Service.