Currently, the web app is quite bare-boned. There is no login feature yet, which means that you cannot use your own playlists and such. It doesn't even have adverts in it.
The web app allows you to listen to any song you please, just like the standard version. The interface has three tabs: Search, Stations and Popular. The aptly-named tabs allow you to search for any song in the Grooveshark library, pick a genre station or look up currently hot songs.
I used it for a while on my iPad (running iOS 5) and it ran pretty well in Safari. Songs even played in the background and they were skip-able using the buttons in the multitasking bar.
The HTML5 app is compatible with iOS devices, Android 2.3+ devices, HP TouchPad, BlackBerry PlayBook and more according to the post on the Grooveshark blog. If you want to help out, you can test the web app on a device not listed in there and contact them.
Visit the HTML5 Grooveshark here: http://html5.grooveshark.com/
