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

Gate One is an HTML5-powered terminal emulator and SSH client

Easily create apps using the web technologies you know and love: HTML, CSS, and JavaScript.
PhoneGap is a free and open source framework that allows you to create mobile apps using standardized web APIs for the platforms you care about.

So here we're collecting all the shims, fallbacks, and polyfills in order to implant html5 functionality in browsers that don't natively support them.

HTML5Forms.js is a JavaScript polyfill that implements a subset of the HTML5
Forms module in all browsers. The script will only add support for the
different parts of the module when there doesn't exist a native
implementation.

Web Forms 2.0 Cross-Browser Implementation

‘320 and Up’ is a lightweight, easy to use and content first responsive web design boilerplate.

HTML5 <video> and <audio> made easy.
One file. Any browser. Same UI.

Streeme can serve music to most HTML5 compliant browsers and smartphone handsets using the computer you already own.

History.js gracefully supports the HTML5 History/State APIs (pushState, replaceState, onPopState) in all browsers. Including continued support for data, titles, replaceState. Supports jQuery, MooTools and Prototype. For HTML5 browsers this means that you can modify the URL directly, without needing to use hashes anymore.

HTML: The Living Standard. A technical specification for Web developers.

LeanBack Player is a Javascript based HTML5 Video and Audio Media Player UI that uses build-in HTML5 <video> and <audio> element functionality of standard browsers, platforms and devices. It's very easy to integrate, it's skinnable by using CSS, it's independent of any other javascript libraries, it's adaptable by using extensions and it's free for non-commercial use.

The HTML5 Shiv enables use of HTML5 sectioning elements in legacy Internet Explorer and provides basic HTML5 styling for Internet Explorer 6-9, Safari 4.x (and iPhone 3.x), and Firefox 3.x.

Compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers.

Impel takes the pain out of working with HTML5 asynchronous databases.

Statement callbacks? Transaction callbacks? One-to-many and many-to-many relationships? How the heck do I deal with all without my head exploding?

Procedural drawing tool

Flowplayer is a beautiful and minimalistic, yet full-featured HTML5 video player that lets your video be the star of the show. Supports all devices.

(X)HTML5 Validator

Using HTML5 and Flash, SoundManager 2 provides reliable cross-platform audio under a single JavaScript API.