Right now, there are three experimental features available: desktop notifications for Chrome, HTML5 audio and 5-star ratings. If you enable 5-star ratings, thumbs ups and thumbs downs will be converted to 5 stars and 1 star. HTML5 audio "works in all browsers that support MP3 playback with HTML5 audio: Chrome, Safari 3.1+, and IE 9+" and replaces the hidden Flash player that's currently used.
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After enabling desktop notifications, you should see an infobar that asks for permission. Click "allow" and Google Music will show a nice notification enhanced with a cover art thumbnail when a new song starts playing.
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{ Thanks, Florian K. }