It's a natural progression for TweetDeck, especially since its originally Adobe Air app is practically all Web code. TweetDeck Web will sport a feature set which is nearly identical to the Chrome app, with the notable exception of Twitter streaming.
Initially, TweetDeck is targeting Firefox 4 and 3.6, Google Chrome, and Safari. Opera and Internet Explorer 9 won't be invited to the dance until a bit later on.
If you'd like to get in on the TweetDeck Web beta, head on over and register -- or sign up using your existing TweetDeck account.