This is a list of known implementations of HTML 5 in web browsers. (It is incomplete, feel free to extend it.)

JSONT is to JSON what XSLT is to XML.

CleverCSS is a small markup language for CSS inspired by Python that can be used to build a style sheet in a clean and structured way. In many ways it's cleaner and more powerful than CSS2 is.

The most obvious difference to CSS is the syntax: it is indentation based and not flat. While this is obviously against the Python Zen, it's nonetheless a good idea for structural styles.

TestSwarm provides distributed continuous integration testing for JavaScript. It was initially created by John Resig as a tool to support the jQuery project and has since moved to become an official Mozilla Labs project.

Pyjamas is a port of Google Web Toolkit to Python.

I'm not a fan of reset style sheets and I won't use this one. But this might be a handy reference, nevertheless.

The geo-location-javascript framework gives access to mobile web users location via javascript. It hides the underlying platform specific implementation through a simple unified interface.

To assist engineers in the development phase it provides a simple mechanism to simulate users locations and movements without being there in the real world.

Modernizr is a small and simple JavaScript library that helps you take advantage of emerging web technologies (CSS3, HTML 5) while still maintaining a fine level of control over older browsers that may not yet support these new technologies.

RED checks HTTP resources to see how they use HTTP, makes suggestions, and finds common protocol mistakes.