The poker tracking software available works by reading hand histories - these vary between the different poker sites, but are generally small text files saved to the user's local appdata folder. There are two problems with this relevant to developing a browser based odds tracker:
- Most browser-based poker rooms do not save hand history locally.
- Even if they do save hand histories locally, you don't have permission to access them from browser-running JavaScript.
A JavaScript solution is possible, but you would have to have a locally running backend, such as a Node.js server, in order to access the hand history files. You could then have API endpoints that a browser-based app could hit to get the data. Conceptually this is little different to the currently available options, just a different technology stack.
The other factor to consider is what technology is being used by the poker room itself. In my experience the majority of these are either Flash or Silverlight - to my knowledge there is no way to easily get data out of these. If you were to find a poker room that had a JavaScript based app, then it is theoretically possible to run a user script on top of this, and respond to certain events in the poker room's script. I say theoretically because this would require fairly intimate knowledge of their code, which will almost certainly have been obfuscated before delivery.
In short, I don't think this is generally feasible.