Modernizr is an open-source JavaScript library that helps you build the next generation of HTML5 and CSS3-powered websites

Use the new and shiny responsibly.
Look up HTML5, CSS3, etc features, know if they are ready for use, and if so find out how you should use them – with polyfills, fallbacks or as they are

When Can I Use tells you the browser support story, while Modernizr gives you the power of feature detection. HTML5 Please helps you out with recommendations for polyfills and implementation so you can decide if and how to put each of these features to use.

The recommendations below represent the collective knowledge of developers who have been deep in the HTML5 trenches. We hope they serve you well.

Google:
HTML5 introduces many cutting-edge features that enable developers to create apps and websites with the functionality, speed, performance, and experience of desktop applications. But unlike desktop applications, apps built on the web platform can reach a much broader audience using a wider array of devices. HTML5 accelerates the pace of your innovation and enables you to seamlessly roll out your latest work to all your users simultaneously.

At Create Digital Media we make beautiful apps for iPhone, iPad and Android. It’s a fine balance between art and science.

Muse is a freeform tool targeted to graphic designers and artists whose work has thus far appeared only in print, but who want to create interactive websites without having to deal with code.

The next-generation Web Editor
based on the rendering engine of Firefox

Komodo 6 Adds HTML5 and CSS3 Support

Learn how to use fluid grids, flexible images, and media queries to create elegant user experiences with responsive web design.

Sass makes CSS fun again. Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.

From advanced selectors to generated content to the triumphant return of web fonts, and from gradients, shadows, and rounded corners to full-blown animations, CSS3 is a universe of creative possibilities.

The professional, open source development tool for the open web
Develop and test your entire web application using a single environment. With support for the latest browser technology specs such as HTML5, CSS3 and JavaScript; and Ruby, Rails, PHP & Python on the server side.

In this tutorial, we are going to build a blog page using next-generation techniques from HTML 5 and CSS 3. The tutorial aims to demonstrate how we will be building websites when the specifications are finalized and the browser vendors have implemented them. If you already know HTML and CSS, it should be easy to follow along.

Last week we launched FindMeByIP.com, a simple app which reveals your browsers' support for CSS3 and HTML5 features in an easy to read format using Modernizr.

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.