A proof-of-concept implementation of "Exchange in a Browser" idea. You trade against an open source code hosted at GitHub Pages. No third party involved, except to provide a bag of private keys + UTXOs to the website when the page loads. The rest happens automatically: if you deposit L-BTC, you get back L-USDt and vice versa. Kind of a large smart contract.
To prove available reserves, balances are computed from the keys. The deposit address is also derived from new private and blinding keys. These keys, along with TxId and Vout are added to the wallet's available UTXOs after funding. This ensures automatic refund in an unlikely event that the purchased asset is not available (some other user purchased all or most of it while your trade was pending).
All trades have blinded amounts and assets, with the order of inputs and outputs randomized. Our private keys are (hopefully!) well protected by encryption and code obfuscation.