An open source web lightwallet was posted on Reddit the source is https://github.com/moneroexamples/openmonero. It would be able to check the balance of your wallet with only the address and view key (not including outgoing/spending). However, it requires a key image to identify which one is the real incoming transaction for that address. The author says that it guesses which are the real incoming transaction. How could this be done? Would this be possible with just client side scripting? Or Does the backend will require a lot of processes and would not scale?