The new Tweetdeck for the Mac is supposedly HTML5-based:
http://www.cultofmac.com/134237/tweetdeck-releases-html5-based-app-in-the-mac-app-store/
Anyone know how
My guess is they do something similar to what PhoneGap does - add a webview in an XCode project. Webviews are basically embedded browsers that can run html5 and javascript. Then, they code their pages in html5 and can call their dataservices via ajax.
Here is actually a tutorial on how to add your html5 code to a webview in xcode: http://www.lostdecadegames.com/how-to-embed-html5-into-a-native-mac-osx-app/
http://nwjs.io/
NW.js lets you call all Node.js modules directly from DOM and enables a new way of writing applications with all Web technologies. It was previously known as "node-webkit" project.
Today you have some great tools made to build Mac OS X applications with HTML5/CSS3 and JavaScript. Like Sentenza Desktop : http://codecanyon.net/item/sentenza-desktop-for-mac/4070984
Check out this http://www.tidesdk.org/ build apps with html5